Coordination Chemistry: Reactions and Mechanisms PDF

COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS The chemistry of coordination compounds is distinctive. • The geometries of complexes are highly diverse and rearrangements are possible. • Metal atoms impose variability on the reactivity of their complexes. Types of Reactions of Coordination Comple...

COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS The chemistry of coordination compounds is distinctive. • The geometries of complexes are highly diverse and rearrangements are possible. • Metal atoms impose variability on the reactivity of their complexes. Types of Reactions of Coordination Complexes: (1) Substitution at the metal center (2) Oxidation–reduction (3) Reactions of the ligands that do not change the attachments to the metal center. (4) Reactions that include more elaborate rearrangements of ligand structures. COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Transition-state theory ⎯ describes chemical reactions as moving from one energy minimum (the reactants) through higher energy structures (transition states, intermediates) 438 Chapter 12 | Coordination Chemistry IV: Reactions and Mechanisms to another energy minimum (the products). Free energy Free energy coordinate diagrams can vary widely, the adopted path between the reactants a • In the generic substitution products is always the lowest energy pathway available and must be the same reg MY + X MX + Y ‡ ¢G MX This + Yis→ + X, of microscopic reversibili of the direction of the reaction. theMY principle lowest energy pathway going begins in one direction mustenergy also be of the lowest energy p the diagram at the free going in the opposite direction. reactants MX and Y as separated species. Extent of reaction The highest energy structure along the reaction pathway is called the tran (a) • As these species encounter each other and 12.1a, the reaction proceeds via a transition state without any stru state. In Figure react, the free energies of the resulting at local energy minima. In Figure 12.1b , such a structure, called an intermed MY + X MX + Y structures change as the M ⎯ X distance MXY formed along the reaction pathway. Intermediates, unlike transition states, are lengthens (the bond breaks) and the M ⎯ Y ¢G‡ times detectable. The presence of undetectable intemediates can allow applica distance shortens (the new bond forms). the steady-state approximation during analysis of the reaction kinetics (Section 1 • Reaction coordinate diagrams usuallytoexhibit Extent of reaction in which the concentration of the intermediate is assumed be extremely sma (b) a saddle shape. essentially unchanging during much of the reaction. FIGURE 12.1 Energy Profiles The activation energy Kinetics experiments are carried out to determine a number of parameters th Energy Profiles and Intermediate Formation. (a) No intermediate. is the energy difference between the reactants and the transition state. (b) An intermediate andminimum Intermediate Formation. (a) energy is present at the small at the top of the curve. The activation is the measured to reaction mechanism. The order of each reactant, indicated by the power at the maximum point of the curve No intermediate. The activation reactant concentration in the differential equation that describes how its concen energy is the energy differchanges with time, indicates how the reaction rate is tied to a change in that rea ence between the reactants reactant concentration in the differential equation that describes how its concentration rchanges with time, indicates how the reaction rate is tied to a change in that reactant’s ts COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS concentration. The rate constant , a proportionality constant that relates the reaction rate b) An the to the concentrations of the reactants, is temperature dependent. By studying a reaction at The adopted path between the reactants and the products is always the lowest energy of different temperatures, the free energy of activation and its components, enthalpy and pathway available and must be the same regardless of the direction of the reaction. energy entropy of activation, can be found. These parameters permit further hypotheses regardum • This is the principle of microscopic reversibility. ing the mechanism and energy changes along the reaction pathway. Examination of pressure dependence on reaction rates provides the volume of activation, which offers insight The highest energy structure along the reaction pathway is called the transition state. into whether the transition state is larger or smaller than the reactants. Even for (local thermodynamically reactions ( !G" 6 0),pathway a large but activation An intermediate minimum) mayfavorable be formed along the reaction unlike energystates, meansare thatsometimes the reactiondetectable. will be slow. For thermodynamically unfavorable reactions transition ( !G" 7 0), even a fast reaction (with small activation energy) is unlikely to occur. The rate Theof rate constant, a proportionality constant relates the reaction rate to the reaction depends on the activation energy,that as in the Arrhenius equation: concentrations of the reactants, is temperature dependent. k = Ae EA - RT or EA ln k = ln A RT Three generic reaction coordinate diagrams are in Figure 12.2 . In (a) and (b), the reac• The rate of reaction also depends on the activation energy. tions have large and positive equilibrium constants since !G 6 0; both reactions are spontaneous. However, in (a), EA is large, so the reaction is slow. Reaction (b) features an intermediate at the energy minimum near the top of the curve. In (c), the reaction can tions have large and positive equilibrium constants since !G 6 0; both reactions are spontaneous. However, in (a), EA is large, so the reaction is slow. Reaction (b) features COORDINATION CHEMISTRY: REACTIONS an intermediate at the energy minimum near the top of theAND curve.MECHANISMS In (c), the reaction can occur quickly because of the low activation energy, but it has a small equilibrium constant because !G 7coordinate 0. Three generic reaction diagrams: ¢EA ¢EA ¢G (a) ¢G (b) ¢EA ¢G (c) FIGURE 12.2 Reaction Coordinate Diagrams. (a), (b) Large activation energy hinders the reaction rate (a),(b) Largeeven activation energyfavors hinders the reaction even though equilibrium though the equilibrium the products since !G < 0;rate (c) Smaller activation energythe facilitates a faster reaction, the equilibrium favors the reactants since !G > 0. In (b), the intermediate is potentially the products sincebut ΔG <0 detectable. favors (b) The intermediate is potentially detectable. (c) Smaller activation energy facilitates a faster reaction, but the equilibrium favors the reactants since ΔG > 0. n+ of substitution reactions involves aqueous metal ions ([M(H2O)m] ) as reactants. These Inert and Labile Compounds reactions can produce colored products used to identify metal ions: COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Many reactions require substitution, replacing one ligand by another. A well-studied class 2+ 2+ n6+H2O [Ni(H O) ] + 6 NH m [Ni(NH ) ] + 2 6 3 3 6 of substitution reactions involves aqueous metal ions ([M(H2O)m] ) as reactants. These 1. Substitution Reactions - replacing a ligand by another bluemetal ions: reactions can producegreen colored products used to identify 12.2.1 3+2+ 2+ 2+ [Fe(H ] + SCN m [Fe(H O) (SCN)] + H O 6 2 5 2 [Ni(H2O) O) ] + 6 NH m [Ni(NH ) ] + 6 H O 2 6 3 3 6 2 verygreen pale violet blue red n+ 3+ 2+rapid and generally form substitution reactions involving [M(H O) ] are [Fe(H2O)6] + SCN m [Fe(H22O)5m(SCN)] + H2O alsovery undergo fast reactions. Addition of HNO pale violet red 3, NaCl, H3PO4, KSCN, and NaF Ligand species that successively to a solution of Fe(NO3)3 # 9 H2O provides an +classic example. The initial soluLigand substitution reactions involving [M(H O) ] 2are + rapid and generally form 2 m tion is yellow because of the presence of [Fe(H O) (OH)] and other Fe(III) complexes n+ 2 5 Ligand substitution reactions involving [M(H O) ] are rapid and generally form species 2 m species that also undergo fast reactions. Addition of HNO , NaCl , H PO , KSCN, and NaF 3+ 3 3 4 containing water and hydroxide ligands derived from the hydrolysis of [Fe(H2O)6] . # that alsosuccessively undergo fast reactions. to a solution of Fe(NO ) 9 H O provides a classic example. The initial solu3 3 2 Although the exact complexes formed and their equilibrium concentrations depend on the 2+ tion is yellow because of the presence of [Fe(H O) (OH)] and other Fe(III) complexes 2 5 of the3ions involved, products representative: Example:concentrations Addition of HNO , NaCl, H3POthese andare NaF in succession to Fe(NO33+)3.9H2O: 4, KSCN, containing water and hydroxide ligands derived from the hydrolysis of [Fe(H2O)6] . 2+ + 3+ [Fe(Hcomplexes + Handh Although the exact their[Fe(H equilibrium 2O)5(OH)]formed 2O)6] concentrations depend on the colorless (very violet) concentrations of the yellow ions involved, these products arepale representative: 2+ [Fe(H2O)5(OH)] 3+ [Fe(H2O)6] yellow + 3+ + H - h [Fe(H2O)6] 2+ + Cl h [Fe(H2O)5(Cl)] + H2O colorless (very pale violet) yellow 3+ 2+ [Fe(H2O)62+ ] + Cl 3-h [Fe(H2O)5(Cl)] [Fe(H2O)5(Cl)] + PO4 h Fe(H2O)5(PO4) yellow + H2-O + Cl 3+ [Fe(H2O)6] - 2+ + Cl h [Fe(H2O)5(Cl)] + H2 O COORDINATION CHEMISTRY: REACTIONS yellow AND MECHANISMS [Fe(H2O)5(Cl)]2+ + PO43- h Fe(H2O)5(PO4) + Clcolorless - Fe(H2O)5(PO4) + SCN h [Fe(H2O)5(SCN)] 2+ + PO4 3- red 2+ [Fe(H2O)5(SCN)] - 2+ + F h [Fe(H2O)5(F)] + SCN - colorless While the fate of these reactions is partially governed by the relative bond strengths Compounds that react rapidly are called labile. between Fe(III) and the incoming and departing ligands, the examination of water exchange • Half-life reactions,of one minute or less is the criterion for lability. n+ 18 called inert or robust. 18 n+ Compounds that react more slowly are [M(H2O)m] + H2 O m [M(H2O)m - 1(H2 O)] + H2O is insightful since the bonds broken and made during the substitution have essentially Labile and inert are kinetic terms, should not be confused with thermodynamic terms identical strengths.* Water exchange rate constants (Table 12.1) vary widely as a function stable and unstable. of the metal ion. • [Fe(H stable (has 12.1 a large Kf), but it is also labile.reactions, Rate like those in Table are known for many substitution 2O)constants 5F]2+ is very and general trends in the speeds of these reactions have been correlated to the electronic 3+ • [Co(NH3)6] is unstable in acid (ΔG < 0), but reacts slowly (kinetically inert). configuration of the starting complex. The rate constants for water exchange differ by 3+ 2+ more than 13 orders of magnitude for [Cr(H2O)6] and [Cr(H2O)6] ! It is also intriguing that [V(H O) ]3 + undergoes water exchange roughly 6 times faster than does [V(H O) ]2 +, 440 Chapter 12 | COORDINATION Coordination Chemistry IV: Reactions and Mechanisms CHEMISTRY: REACTIONS AND MECHANISMS n+ n+ TABLE Rate Constants for Water Exchange in [M(H O) ] Rate12.1 Constants for Water Exchange in [M(H 2 6 2O)6] Complex k(s−1) (298 K) Electronic Configuration* [Ti(H2O)6]3+ 1.8 * 105 t2g 1 [V(H2O)6]3 + 5.0 * 102 t2g 2 [V(H2O)6]2 + 8.7 * 101 t2g 3 [Cr(H2O)6]3 + 2.4 * 10 - 6 t2g 3 [Cr(H2O)6]2+ 7 108 t2g 3eg 1 [Fe(H2O)6]3 + 1.6 * 102 t2g 3eg 2 [Fe(H2O)6]2 + 4.4 * 106 t2g 4eg 2 [Co(H2O)6]2 + 3.2 * 106 t2g 5eg 2 [Ni(H2O)6]2 + 3.2 * 104 t2g 6eg 2 [Cu(H2O)6]2 + 4.4 * 109 t2g 6eg 3 [Zn(H2O)6]2 + 7 107 t2g 6eg 4 *These configurations assume octahedral geometry, even in cases where Jahn–Teller distortion is anticipated. Data from R. B. Jordan, Reaction Mechanisms of Inorganic and Organometallic Systems, 3rd ed., Oxford (New York), 2007, p. 84. • The rate constants for water exchange differ by more than 13 orders of magnitude for [Cr(H2O)6]3+ and [Cr(H2O)6]2+. • even though ligand loss from a V(III) ion might be expected to be more difficult than from a [V(H2O)6]3+ undergoes exchange roughly 6 times faster [V(H2O)6]2+. V(II) ionwater on the basis of an electrostatic argument. Complexes such asthan [Cr(H2does O)6]2 + , which react rapidly, essentially exchanging one ligand for another within the time of mixing the reactants, are classified as labile. A labile complex has a very low activation energy for ligand substitution. Taube1 suggested a reaction half-life (the time it takes for the concentration of the initial compound to decrease by one half) of one minute or less as the criterion 3+ General rules: 4 [Cr(H2O)6] undergoes water exchange exceedingly slowly relative to the high-spin d [Cr(H2O)6]2 + , and [V(H2O)6]2 + reacts slower than [V(H2O)6]3 + . With strong-field ligands, 8 COORDINATION REACTIONS AND d atoms often form inertCHEMISTRY: square-planar complexes. Compounds withMECHANISMS other d configurations tend to be labile, with a wide range of substitution reaction rate constants. Slow Reactions (Inert) Moderate Rate d 3, low-spin d 4, d 5, and d 6 8 Strong-field d (square planar) Fast Reactions (Labile) d 1, d 2, high-spin d 4, d 5, and d 6 Weak-field d 8 7 9 d ,d ,d 10 Mechanisms of Substitution MECHANISMS12.2.2 OF SUBSTITUTION Free energy Free energy Langford and Gray2 described a range of possibilities for substitution reactions (Table 12.2). onethe extreme, the departing ligand leaves, and and anan intermediate with a lower Dissociation, DAt⎯ departing ligand leaves intermediate withcoordinaa lower tion numberisisformed formed, a mechanism labeled D for dissociation. At the other extreme, the coordination number incoming ligand adds to the complex, and an intermediate with an increased coordination is incoming formed in a mechanism labeledtoA the for association . Between theintermediate two extremes Association, Anumber ⎯ the ligand adds complex and an with an is interchange , I , in which the incoming ligand assists in the reaction, but no detectable increased coordination number is formed. M+X intermediates appear. When the degree of assistance is small and the reaction is pri+Y dissociative, ⎯ it isAcalled interchangeintermediate , Id . When the incoming ligand mechanisms or Ddissociative with detectable • Intimatemarily M MX + Y begins forming a bond to the central atom before the departing ligand bond is weakened it is called associative interchange , Ia . Many reactions are described by assists Ia Extent of reaction Interchange, Iappreciably, ⎯ between the two extremes (A and D); the incoming ligand in the (a) or Id mechanisms, rather than by A or D, when the kinetic evidence points to association reaction but no intermediate is formed or detected or dissociation, but detection of intermediates is not possible. The categories D, A, and I MXY are called the stoichiometric mechanisms; the distinction between activation processes that are associative and dissociative is called the intimate mechanism. The similarities M in the energy profiles for associative and dissociative reactions (Figure 12.3) show that MX + Y COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Dissociative interchange, Id ⎯ the degree of assistance is small and the reaction is primarily dissociative Associative interchange, Ia ⎯ the incoming ligand begins forming a bond to the central atom before the departing ligand bond is weakened appreciably Stoichiometric mechanisms ⎯ the categories A, D, or I 12.3 Kinetic Consequences of Reaction Pathways | 441 Classification Substitution Mechanisms forOctahedral Octahedral Complexes TABLE 12.2 Classifiof cation of Substitution Mechanisms for Complexes Stoichiometric Mechanism Intimate Mechanism Dissociative activation Associative activation Associative 7-Coordinate Intermediate for Octahedral Reactant Dissociative 5-Coordinate Intermediate for Octahedral Reactant D Id A Ia electronic structures. These configurations have no electrons in the eg orbitals, and at least one electron in each t2g orbital. The d3 inert classification is exhibited in Table 12.1; 3+ 4 and d a Free energy Free energy Free energy anism labeled D for dissociation. At the other extreme, the mplex, and an intermediate with an increased coordination COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS nism labeled A for association. Between the two extremes The similarities the energy profiles for and dissociative reactions show that e incoming ligand in assists in the reaction, butassociative no detectable unambiguously distinguishing between these mechanisms can be challenging. M+X he degree of assistance is small and the reaction is priable 12.2). +Y dcoordinadissociative interchange, Id . When the incoming ligand MY + X MX + Y central atom before the departing ligand bond is weakened Energy Profile for Associative Mechanism treme,Energy the Profile for Dissociative Mechanism ative interchange , I . Many reactions are described by I Extent of reaction a a ordination (a) by A or D , when the kinetic evidence points to association extremes of intermediates is not possible. The categories D , A , and I detectable MXY mechanisms ; the distinction between activation processes M + X on is pri+ Y mechanism. The similarities ciative is called the intimate ing ligand MY + X MY + X ociative and dissociative reactions ( Figure 12.3 ) show that MX + Y MX + Y weakened between these mechanisms can be challenging. ibed by I Extent of reaction Extent of reaction (a) ree energy ssociation D , A , and I quences of Reaction Pathways MXY processes ples in which the rate law is used to propose reaction milarities types of information: (1) the information MY + X used to propose (b) FIGURE 12.3 Energy Profiles for Dissociative and Associative Reactions. (a) Dissociative mechanism. The intermediate has 5 5 amination of the possibility of a D mechanism typically requires the steady-state k2 lower coordination number. Subsequent additions of either a new ligand (Y) or the leaving ML5 + Y h ML5Y mation . This approximation assumes that a vanishingly small (and constant) conration 12 |of Coordination Chemistry IV: Reactions and Mechanisms the possibility a D mechanism the steady-state group (X) are twoofpossible reaction typically pathways requires for this intermediate: on. This of theapproximation intermediate, ML5, isthat present during reaction by assuming that the MECHANISMS COORDINATION CHEMISTRY: REACTIONS AND on assumes a vanishingly small (andExamination constant) conk1 the of the possibility of a D mechanism typically requ MLthe MLby5equal. + X If these 5X m formation and consumption of the intermediate are are the the intermediate, ML5, is present during reaction assuming that. rates the approximation approximation This assumes that a vanishingly sma k-1 Dissociation ( D ) he change in the [ML ] must equal zero (and this species cannot accumulate) during ML5, is present during the reaction mation and consumption of the intermediate are Ifcentration these rates kequal. 5 (D) 2 DISSOCIATION of are thethe intermediate, ML +species Y h ML Ystep is loss during Inaa rate dissociative (Dthis )5reaction, the first of a ligand to form an intermediate with a 5accumulate) ange inExpressed the [ML5] as must equal zero (and cannot ction. equation, rates of formation and consumption of the intermediate are equal. The loss of a ligand to form an intermediate with a lower coordination number is followed by lower coordination number. Subsequent additions of either a new ligand (Y) or the leaving Expressed as a rate equation, Examination of the possibility of a D mechanism typically requires the steady-state same, the change in the [ML ] must equal zero (and this species canno d[ML ] 5 5 addition of=group ak new ligand to the intermediate: (X) are two possible reaction pathways for this intermediate: [ML X] k [ML ][X] k [ML ][Y] = 0 1 5 1 5 2 5 d[ML approximation . This approximation assumes that the a vanishingly small (andas constant) conreaction. Expressed a rate equation, 5] dt = k1[ML5X] - k - 1[ML5][X] - k2[ML5][Y] = 0 k1 centration of the intermediate, ML5, is presentML during the reaction by assuming that the dt X m ML + X 5 5d[ML5] for [ML 1 rates5],of formation and consumption of the intermediatek -are equal. If these are the- k - 1[ML5][X] - k2[ML5][Y = rates k1[ML 5X] dt ML5],same, the change in the [ML ] must k2 k [ML X] 1 equal 5 zero (and this species cannot accumulate) during 5 ML + Y h ML Y [ML5] k= 5 5 [ML X] 1 rate 5equation, the reaction. Expressed as a k - 1[X] + k2[Y] Solving for [ML5], [ML5] = k - 1[X] +ofkthe 2[Y]possibility of a D mechanism typically requires the steady-state Examination k [ML X] d[ML ] 1 5 5 stituting into the rate law for=formation of the product, [ML(and k1. [ML - k - 1[ML5][X] - k2[ML = 0 5] = constant) conapproximation This approximation assumes that5][Y] a vanishingly small ing into the rate law for dt formation of the5X] product, k - 1[X] + k2[Y] d[ML Y] intermediate, ML5, is present during the reaction by assuming that the centration of 5the d[ML5Y] = k [ML ][Y] and substituting into the rate law for formation of the product, 2 5 Solving for [ML rates5],of formation and consumption of the intermediate are equal. If these rates are the = k2[ML5][Y] dt dt k [ML X] 1 5 same, the change in the [ML ] must equal zero (and this species cannot accumulate) during the rate law: d[ML Y] 5 5 [ML5] = ate law: = k2[ML5][Y] + k2[Y] the reaction. Expressed as akrate equation, - 1[X] dt • Evidence that a D mechanism may be operative, includes an d[ML k Y] k [ML X][Y] 1 5 d[ML5Y] 5 k2k1= [ML25X][Y] dependence and substituting rate d[ML law for theinverse product, Rate law: into the to the rate law: between the rate of formation of [ML5Y] 5] formation of leads = dt k - 1[X] k+- 1k[X] + k [Y] = k [ML k -the ][Y] = 0 in the rate law. 2 dt [Y] and concentration of X5as indicated 1 5X] 1[ML 5][X] - k2[ML 2 dtd[ML5Y] d[ML k Y] k [ML X][Y] 5 2 1 5 idence that the conditions associated the steady-state approximation are pres= kapproximation [ML ][Y] e that the conditions associated with thewith steady-state are pres= 2 5 dt dt k [X] + k [Y] Solving for [ML ], 1 2 dathat a D mechanism may be operative, includes an inverse dependence between 5 D mechanism may be operative, includes an inverse dependence between leads to the rate law: k [ML X] of formation of5Y] [ML Y] and the concentration of X as indicated in the rate law 1 5 ormation of [ML and the concentration of X as indicated in the rate law 5 Evidence that the conditions associated with the steady-state app 12.3.1 than Y reacts with ML5. A D mechanism on the basis of the derived rate law will exhibit a complicated depenCOORDINATION REACTIONS AND MECHANISMS dence upon [X] and [Y] whichCHEMISTRY: has two limiting cases described below. Studies that systematically vary the concentrations of both X and Y provide the best evidence for a dissociative Studies mechanism. that systematically vary the concentrations of both X and Y provide the best At high [Y], the system will show saturation kinetics in which the reaction evidence a dissociative ratefor depends only upon mechanism. [ML5X]. d[ML5Y] k2k1[ML5X][Y] d[ML5Y] = and = k1[ML5X] dt k - 1[X] dt If [X] W [Y] so that k-1[X] W k2[Y] If [Y] W [X] so that 12.3 Kinetic Consequences of Reaction k2[Y] W k-1[X] • At high [Y], the system will show saturation kinetics in thesewhich relationships support both theonly validity of 5X]. the reaction rate depends upon [ML While(I)experimental data that exhibit INTERCHANGE Interchange (I) and a D substitution mechanism, the exceedingly low [ML5] the steady-state approximation I is a direct replacement of theinleaving group with the incoming group that does not An interchange ( I ) reaction its simplest form is a direct replacement of the leaving group concentration that forms on the basis of this approximation renders the detection of this proceed via an intermediate, butdoes rather aproceed single transition state leading to the conversion with the incoming group that not via an intermediate, but rather a single intermediate impossible in many cases. of reactants to state products. transition leading to the conversion of reactants to products. 12.3.2 k1 ML5X + Y h ML5Y + X If the substitution reaction is irreversible, an interchange mechanism will exhibit second order kinetics, being first order in [ML5X] and first order in [Y]. d[ML Y] transition state leading to the conversion of reactants to products. k1 transition state leading to the conversion of reactants to products. ML5X + Y h ML Y + X 5 k1 COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS ML5X + Y h ML5Y + X If the substitution reaction is irreversible, an interchange mechanism will exhibit second If the substitution reaction is irreversible, an5X] interchange mechanism will exhibit Iforder the substitution reaction is irreversible, an interchange mechanism will exhibitsecond second order kinetics, being first order in [ML and first order in [Y]. kinetics, being first order [MLorder first5order [Y].order in [Y]. 5X] and order kinetics, beinginfirst in [ML X] andinfirst d[ML5Y] = k [ML X][Y] 1 5 d[ML 5Y] dt Rate law: = k1[ML5X][Y] If the interchange reaction is reversible, a more complicated treatment is required to deal dt the approachreaction towardsisthe resultingaequilibrium. A common experimental condition Ifwith the interchange reversible, more complicated treatment is required to deal is tothe employ high towards [Y] and [X] resulting to approximate the reaction as a pair pseudoIf the interchange reaction is reversible, approximate the reaction asofaopposing pair of condition opposing with approach the equilibrium. A common experimental order pseudo-first orderreactions. reactions. isfirst to employ high [Y] and [X] to approximate the reaction as a pair of opposing pseudok1 first order reactions. ML X + Y m ML Y + X 5 kk1- 1 5 ML5X + Y m ML5Y + X d[ML5X] d [ML5Y] k - 1 = = k1[ML5X] - k-1[ML5Y] d[ML d [ML dt5X] dt5Y] = = k1[ML5X] - k-1[ML5Y] dt dt and [Y] are large If [X] * If [X] and [Y] are large Espenson has provided further details regarding this kinetics treatment. The I and D • increasing [Y] will not lead to saturation kinetics with I unlike when a D mechanism is operative. mechanisms can be differentiated in principle since increasing [Y] will not lead to satura* Espenson has provided further details regarding this kinetics treatment. The I and D tion kinetics with I as predicted when a D mechanism is operative. mechanisms can be differentiated in principle since increasing [Y] will not lead to saturaTwo variations on the interchange mechanism are Id, dissociative interchange, and Ia, Espenson has provided further details regarding this kinetics treatment. The I and D Two variations on the interchange mechanism are Id, dissociative , and Ia, mechanisms can be differentiated in principle since increasing [Y] will notinterchange lead to saturaassociative interchange . The difference is the relative strengths of the M—X and M—Y tion kinetics with I as predicted when a D mechanism is operative. COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS bonds the transition If bonding betweenare theIdincoming ligand and the metal Two in variations on thestate. interchange mechanism , dissociative interchange , and Iisa, more important in the transition state, it is isanthe Ia relative mechanism. If breaking the bond Two variations ofinterchange Interchange: associative . The difference strengths of the M—X andbetween M—Y the departing ligand and the metal is more important in the transition state, it is an I mechanism. bonds in the transition state. If bonding between the incoming ligand and the metal is more d Dissociative Interchange, Id : M ⎯ X bond is more important in the T.S. important in the transition state, it is an Ia mechanism. If breaking the bond between the Associative Interchange, I a : M ⎯ Y bond is more important in the T.S. departing ligand and the metal Association (A)is more important in the transition state, it is an Id mechanism. In an associative reaction, forming an intermediate with an increased coordination number ASSOCIATION (A) Association (A)step. This first step is followed by a faster reaction in which the is the rate-determining an associative forming an intermediate with an increased coordinationnumber, number is the rateexiting ligand reaction, isan lost:intermediate The firstInstep, forming with an increased coordination k1 is the rate-determining step. This first step is followed by a faster reaction in which the determining step. It is followed by aML faster reaction in which the leaving ligand is lost: 5X + Y m ML5XY k-1 exiting ligand is lost: 12.3.3 12.3.3 kk21 MLML + Yh m ML ML55Y XY+ X 5X 5XY k -1 The steady-state approximation, whichk2assumes [ML5XY] to be exceedingly small, results ML5XY h ML5Y + X in second-order kinetics regardless of the concentration of Y: The steady-state approximation, which assumes [ML5XY] to be exceedingly small, results d[ML k Y] k [ML X][Y] 5 1 2 5 in second-order kinetics regardless of the concentration of Y: = = k[ML5X][Y] Rate law: dt k - 1 + k2 d[ML5Y] k1k2[ML5X][Y] = = k[ML5X][Y] E X E R C I S•Ean12A.1 dt k - 1 detection + k2 mechanism requires of the [ML5XY] intermediate. the.1preceding equation is the result of the steady-state approximation for an E XShow E R C Ithat S E 12 associative reaction. same or larger than those for square-pyramidal transition states. These calculations give insight in regard to which electronic configurations in octahedral complexes result in lower COORDINATION CHEMISTRY: MECHANISMS barriers for REACTIONS ligand dissociation.AND While other factors are required to understand the kinetics of dissociative substitution reactions, the LFAE values correlate reasonably well with Section 12.1.1 classifications. The highest (most positive) LFAE values are associated Experimental Evidence in Octahedralthe Substitution with d3 and low-spin d4, d5, and d6; all of these ions are classified as inert, supported by a) Dissociation the predicted relatively high activation barriers for ligand dissociation from octahedral complexes of these ions. The negative LFAE parameters predict low activation barriers for • an octahedral complex loses onedissociation, ligand (X) a 5-coordinate transition state, and and to are yield associated with ions classified as labile. This approach fares poorly 8 ions, but these the incoming ligand ultimately fills with thedvacant siteions to generally form the octahedral do not form octahedralproduct. complexes. • The splitting of d-based levels from Oh to SQP (C4v) is a function of the ligands and the extent with which the four ligands that comprise the square of the pyramid bend away from the axial ligand. x2-y2 E (eg, 0.600 ¢o) z2 (b1, 0.914 ¢o) x2-y2 z2(a1, 0.086 ¢o) 0 xy (t2g, - 0.400 ¢o) xy xz Oh yz xz (b2, - 0.086 ¢o) yz (e1, - 0.457 ¢o) C4v Change in energy levels (in terms of Δo) upon ligand dissociation *Although quantifying these changes in orbital energies is beyond the scope of this text, it is reasonable that the from octahedral ML6 to square-pyramidal ML5. dz2 orbital should become less antibonding upon removal of a ligand along the z-axis. This orbital changes from eg to a1 symmetry upon ligand dissociation, and subsequently engages in mixing with the pz and s orbitals as well further decreasing its energy. In strong field C4v complexes, electrons occupy the lower four orbitals singly for d1–d4 configurations, then doubly, with the b1 orbital unoccupied until a d 9 configuration is reached. COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Ligand field activation energy (LFAE) ⏤ the difference between the LFSE of the SQP TS and the LFSE of the octahedral reactant. 446 Chapter 12 | Coordination Chemistry IV: Reactions and Mechanisms Ligand Field Activation Energies TABLE 12.3 Ligand Field Activation Energies Strong Fields (units of ! o ) Weak Fields (units of ! o ) System Octahedral LFSE Square-Pyramidal LFSE LFAE Octahedral LFSE Square-Pyramidal LFSE LFAE d0 0 0 0 0 0 0 d1 −0.400 −0.457 −0.057 −0.400 −0.457 −0.057 d2 −0.800 −0.914 −0.114 −0.800 −0.914 −0.114 d3 −1.200 −1.000 0.200 −1.200 −1.000 0.200 d4 −1.600 −0.914 0.686 −0.600 −0.914 −0.314 d5 −2.000 −1.371 0.629 0 0 0 d6 −2.400 −1.828 0.572 −0.400 −0.457 −0.057 d7 −1.800 −1.914 −0.114 −0.800 −0.914 −0.114 d8 −1.200 −1.828 −0.628 −1.200 −1.000 0.200 d9 −0.600 −0.914 −0.314 −0.600 −0.914 −0.314 d10 0 0 0 0 0 0 For a square-pyramidal transition state, LFAE = square pyramid LFSE – Octahedral LFSE, for s donor only. Electronic and steric factors also influence substitution reaction rates of octahedral complexes. The inequalities below indicate relative rates for ligand exchange via presumed dissociative mechanisms. • The highest (most positive) LFAE values are associated with d3 and low-spin d4, d5, and d6; all of these ions are classified as inert. • The negative LFAE parameters predict low activation barriers for dissociation, and are associated with ions classified as labile. al transition state, LFAE = square pyramid midal transition state, LFAE = square pyramidLFSE LFSE– –Octahedral OctahedralLFSE, LFSE, for for s s donor donor only. only. COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Electronic and steric factors also influence substitution reaction rates of octahedral complexes. Electronic substitutionreaction reactionrates ratesofofoctahedral octahedral Electronicand andsteric steric factors factors also also influence substitution complexes.The Theinequalities inequalitiesbelow below indicate indicate relative complexes. relative rates ratesfor forligand ligandexchange exchangevia viapresumed presumed Metal ion dissociative factors that affect reaction rates of octahedral complexes mechanisms. dissociative mechanisms. 1. Oxidation state of the central ion. Oxidationstate stateofofthe the central central ion. ion. Central atoms 1.1.Oxidation atoms with with higher higheroxidation oxidationstates stateshave have slower ligand exchange rates. states have slower ligand exchange rates. • Central slower ions with higher oxidation ligand exchange rates. 33[AlF66]] [AlF 3+ 3+ + + [Na(H O) ] 2 n [Na(H2O)n] 1+ 1+ 7 7 7 7 22[SiF ] 6 [SiF6] 4+ 4+ 7 7 -[PF [PF66]] 5+ 5+ 2+ 2+ [Mg(H O) ] 2 n [Mg(H2O)n] 2+ 2+ 77 77 SF SF66 6+ 6+ 3+ 3+ [Al(H O) ] 2 6 [Al(H2O)6] 2. Ionic radius. 2. Ionic radius. Smaller ions have slower exchange rates. 2. Ionic radius. Smaller ions have slower exchange rates. • Smaller ions have slower exchange rates. 2+ [Sr(H2O)6]2+ [Sr(H2O)6] 112 pm 112 pm 7 7 2+ [Ca(H2O)6] 2+ [Ca(H2O)6] 99 pm 99 pm 7 7 3+ 3+ 2+ [Mg(H2O)6] 2+ [Mg(H2O)6] 66 pm 66 pm These two trends are attributed to a higher electrostatic attraction between the cen•Both effects These can betwo attributed toare a higher electrostatic attraction between the central atom and the attached ligands. trends attributed to a higher electrostatic attraction between the central atom and the ligands. A strong mutual attraction will slow a reaction occurring via tral atom and the ligands. A strong mutual attraction will slow a reaction occurring via a dissociative mechanism, because bond breaking between the metal and the departing a ligand dissociative mechanism, because bond breaking between the metalsmall and the is required in the rate determining step. Despite the relatively sizesdeparting of the 2+ COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Evidence that supports a dissociative mechanism: 1. The rate of reaction changes only slightly with changes in the incoming ligand. • In many cases, aquation (substitution by water) and anation (substitution by an anion) rates are comparable. • The entering group should have no effect on the reaction rate. 2. Making the charge of the reactant complex more positive decreases the rate of substitution. • The electrostatic attraction between M ion and L electrons increases as the charge of the complex becomes more positive, decreasing the rate of ligand dissociation. COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Evidence that supports a dissociative mechanism: 3. Steric crowding on the reactant complex increases the rate of ligand dissociation. • When ligands on the reactant are crowded, loss of one of the ligands is made easier. On the other hand, if the reaction has an A or Ia mechanism, steric crowding interferes with the incoming ligand and slows the reaction. 4. The rate of reaction correlates with the metal–ligand bond strength of the leaving group, in a linear free-energy relationship (LFER). 5. Dissociative mechanisms generally result in positive values for volume of activation Vact • In D mechanism one species splits into two in the rate determining step. • A mechanisms have negative Vact because two species combine into one. ges. Dissociative mechanisms generally result in positive values for !Vact because species splits into two in the rate determining step, and associative mechanisms t in negative values because two species combine into one in this step, with a COORDINATION CHEMISTRY: REACTIONS umed transition state volume smaller than the total for the reactants. Caution is ed in interpreting volume effects to account for solvation effects, particularly for Linear Free-Energy Relationship y charged ions. AND MECHANISMS • observed when the bond strength of M ⎯ L bond (thermodynamic parameter) plays a major role Relationships in determining the dissociation rateChemistry of aIV: Reactions ligand (kinetic parameter) 448 Chapter 12 | Coordination and Mechanisms Linear Free-Energy log k ffects are related to thermodynamic effects by linear FIGURE free-energy relationships 12.5 Linear Free Energy Relationship for [Co(NH3)5bond X]2+ A LFERWhen can be observed when the bond strength of a metal-ligand (correat 25.0 °C (X indicated LFER is true, a plot of the logHydrolysis of the at each point). Thedissociation log of the rate thermodynamic parameter) plays a major role in determining the rate n+ + Y substitution constant is plotted against the rate constants for [ML X] 5 d (correlated to a kinetic parameter). When this is true, alogplot the logarithm of the of theof equilibrium constant n+ for the acid hydrolysis reaction reactions, where X is varied but Y is not, ants for [ML X] + Y substitution reactions, where X is varied but Y is not, ver5 of [Co(NH3)5X]2+ ions. Data for S. C. Chan,[ML J. Chem. Soc. , m+ + X versus the log constants of the equilibrium garithm of the equilibrium for [ML5X]n + constants +F– from Y m Y] 5 1964, 2375, and for I– from R. G. m+ + X is Yalman, The Arrhenius temperature of rate constants Inorg. Chem. , 1962, 1, 16. and the for [MLequation Y⟺ [ML5Y]dependence linear. 5X]n+ +for All other data from A. Haim, H. for temperature dependence of equilibrium constants Taube, justify correlation. Inorg. this Chem., 1964 , 2, 1199. EA ln k = ln A RT kinetic and - !H" !S" ln K = + RT R thermodynamic -4 -4.5 NO3- -5 Br- -5.5 I- Cl- -6 H2PO4- -6.5 -7 -7.5 F1.5 -1 -0.5 0 log Keq 0.5 1 1.5 Linear Free Energy Relationship for [Co(NH 3)5X]2+ Hydrolysis at 25.0 °C 10 Langford argued that X is dissociated and acts as a solvated anion in the transi- e preexponential factor, A, and the entropy change, !S", are very similar for for [Co(NH3)5X]2 + hydrolysis, and that water is, at most, weakly bound in the tion state transition of substitution reactions considered, and the activation energy, EA, depends on state. A stronger bond between a metal and a leaving group Examples resultsofintheavarying larger activation energy, impacts of incoming ligands are givenainlogical Tables 12.4 and lpy of reaction, !H", there will be a linear correlation between ln k and12.5 ln. K . Table 12.4 data were obtained under pseudo-first order conditions (large [Y]).* The connection for a dissociative mechanism. k1 column gives the rate constants for anion exchange; the k1/k1(H2O) shows the ratio t line on such a log–log plot is indirect evidence for a strong influence The of the of k1 to the rate for water exchange. The rate constants do not vary significantly with the namic parameter, !H", on the activation energy of the reaction. A strongersubstituting bond anion, as would be expected for a dissociative mechanism. a metal and a leaving group results in a larger activation energy, a logical conTable 12.5 gives data for the second-order region for anation of [Ni(H2O)6]2 +, which ** The second-order rate conis hypothesized to occur via a preassociation mechanism. or a dissociative mechanism. Figure 12.5 shows an example from the hydrolysis nucleophile relative to [Cr(H2O)6]3 + . The very large absolute differences in these rate constants are intriguing. The varying amount of electron density at the Cr(III) centers of these CHEMISTRY: complexes due to theCOORDINATION increased donation of ammonia relative to waterREACTIONS plays a significantAND role in determining both the substitution mechanism and the reaction rate. some cases, the Mechanism typical substitution- less mechanism for a complex varies with the b)InAssociative common with O complexes h metal oxidation state. For example, reactions of Ru(III) compounds frequently have associative interchange mechanisms, and those of Ru(II) compounds generally have Effects of Entering Groups on Rates TABLE 12.6 Effects of Entering Group on Rates Rate Constants for Anation [Cr(NH3)5(H2O)]3+ Entering Ligand k(10−4 M−1 s−1) [Cr(H2O)6]3+ k(10−8 M−1 s−1) NCS - 4.2 180 NO3 - i 73 Cl - 0.7 2.9 - 3.7 0.9 I- i 0.08 CF3COO - 1.4 i Br Data from D. Thusius, Inorg. Chem., 1971, 10, 1106; T. Ramasami, A. G. Sykes, Chem. Commun., 1978, 378. MECHANISMS • In the case of water substitution by L anions in [Cr(NH3)5(H2O)]3+, the rate constants are similar (within a factor of 6), indicative of an Id mechanism. • The same ligands reacting with [Cr(H2O)6]3+ show a large variation in rates (more than a 2000-fold difference), indicative of an Ia mechanism. The varying electron density at the Cr(III) centers due to the increased donation of NH3 relative to water plays a significant role in determining both the substitution mechanism and the reaction rate. charge may reduce the RuiH2O bond strength enough to promote an Id mechanism. CHEMISTRY: REACTIONS AND MECHANISMS 12.4.4COORDINATION The Conjugate Base Mechanism Some cases in which second-order kinetics suggest an associative mechanism are believed The Conjugate Base to proceed via aMechanism conjugate base mechanism,12 called SN1CB for substitution, nucleophilic, 13 Some associative mechanism are believed to proceed a conjugate base unimolecular, conjugate base. These reactions depend onvia amine, ammine (NH aqua 3), ormechanism, thatsubstitution, can be deprotonated to form amido or hydroxo species that subsequently release called Sligands nucleophilic, unimolecular, conjugate base. N1CB for ligand dissociatively. is oftenammine the ligand trans to aqua the amido or hydroxo group that dis• These areactions depend onItamine, (NH ligands that can be deprotonated 3), or Octahedral seem particularly to this mechanism. to formsociates. amido or hydroxo Co(III) speciescomplexes that subsequently releasepredisposed a ligand dissociatively. Thethe small radiustrans of low-spin Co(III) or may render itgroup sufficiently Lewis acidic to stabilize the • It is often ligand to the amido hydroxo that dissociates. five coordinate intermediate via p interactions. Example: 2+ [Co(NH3)5X] + OH [Co(NH3)4(NH2)X] + 2+ [Co(NH3)4(NH2)] - m [Co(NH3)4(NH2)X] 2+ h [Co(NH3)4(NH2)] + + X + H2 O (equilibrium) (1) - (slow) (2) (fast) (3) 2+ + H2O h [Co(NH3)5(OH)] Overall, 2+ [Co(NH3)5X] + OH - 2+ h [Co(NH3)5(OH)] TABLE 12.7 Rate Constants for [Ru(III)(EDTA)(H O)]− Substitution + X - a trigonal-bipyramidal geometry (with only slightly widening of two N—Co—N angles than does the Figure 12.6b reaction, which requires more dramatic rearrangement of a square-pyramidal structure. The activation energy associated withAND hydrolysis of the square COORDINATION CHEMISTRY: REACTIONS MECHANISMS pyramidal intermediate in Figure 12.6b, where the hydroxo ligand would engage an orbital orthogonal to the amido nitrogen, is sufficiently high that the (slow) rearrangement to a 2+ Reactions with [Co(tren)(NH3)Cl] isomers show that the nitrogen atom trans to the trigonal-bipyramidal structure, followed by hydrolysis is still the faster pathway. Both of leaving group is these most likely to be deprotonated in the conjugate base mechanism. routes are much slower than that in Figure 12.6a . H2N H2N N Co NH2 - H+ NH3 Cl HN H2N N Co NH2 - ClCl HN H2N NH3 (a) H2N H2N N Co Cl NH2 - H+ NH3 HN H2N N Co Cl NH2 - Cl- Base Hydrolysis of [Co(tren)(NH3)Cl]2+ Isomers. (a) Leaving group (Cl−) trans to deprotonated nitrogen. (b) Leaving group (Cl−) cis to deprotonated nitrogen. Co H2N H2N NH3 HN N Co SQP (b) NH2 +H2O N Co NH2 OH NH3 TBP H2N NH3 N NH2 +H2O NH3 H2N H2N +H N Co H2N OH NH3 2O H2N NH2 N Co OH 85% NH2 NH3 15% FIGURE 12.6 Base Hydrolysis of [Co(tren)(NH3)Cl]2+ Isomers. (a) Leaving group (Cl−) trans to deprotonated nitrogen. (b) Leaving group (Cl−) cis to deprotonated nitrogen. (Data from D. A. Buckingham, P. J. Creswell, A. M. Sargeson, Inorg. Chem., 1975, 14, 1485.) (Figure 12.7). 12.4.5 atalyzed COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS The Kinetic Chelate Effect s of The chelate effect (Section 10.1.1) causes polydentate complexes to be thermodynamically complex The Kinetic Chelate Effect d via a more stable than their monodentate counterparts.17 Substitution for a chelated ligand is The chelate effect causes polydentate complexes to be thermodynamically more hanism generally a slower reaction than that for a similar monodentate ligand. Explanations for their monodentate counterparts. the blue stable than this effect center on two factors. First, the !H associated with removal of the first bound atom is larger than for a related monodentate ligand. If this atom does separate from the Substitution for a chelated ligand is generally a slower reaction than that for a similar metal center, its kinetic barrier for subsequent reattachment is lower than for a related monodentate ligand. monodentate ligand since the former remains in close proximity to the metal center.18 Δ︎H associated with removal of the first bound atom is larger than for a related monodentate • the Consider the general scheme below: ligand. NH2 CH2 M NH2 CH2 slow fast NH2 M NH2 CH2 CH2 (1) OH NH 2 NHsubsequent • If this atom does separate from2 the metal center,fast its kinetic barrier for reattachment is 2 M CH2 + ligand lower than for a related monodentate former remainsCH in close proximity to the metal H2O since the M (2) 2 center. NH2 CH2 NH2 CH2 This kinetic chelate effect dramatically reduces aquation reaction rates. OH2 NH2 OH2 slow M CH2 (3) M + NH2CH2CH2NH2 NH CH COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Stereochemistry of Reactions Dissociative mechanisms lead to products where the stereochemistry may be the same or 454 Chapter | Coordination Chemistry IV: Reactions and Mechanisms different than the 12 starting complex. e.g. H2N Cl Cl Co NH2 ¶ H2N Cl Cl Co H2N ¶ NH2 +OH- H2N NH2 1dil2 Cl Cl Co NH NH2 H2N -Cl- NH2 Cl Conjugate base mechanism NH2 +OH- NH2 1conc2 H2N Cl Cl Co H2N H N H H N H OH- Co NH NH2 +OH- H2N NH2 (dil) OH Co Cl NH NH2 +H+ OH H2N NH2 Co Cl NH2 NH2 ¶ (a) -Cl- NH2 H2N Cl Co H2N H N H O H N H H water (b) H2N H2N Cl Co NH2 NH2 OH ¢ FIGURE 12.8 Mechanisms of Base Hydrolysis of !-cis-[Co(en)2Cl2]+. (a) Retention of configuration in dilute hydroxide. (b) Inversion configuration in concentrated hydroxide. Mechanisms ofofBase Hydrolysis of L-cis-[Co(en) 2Cl2]+. (a) Retention of configuration in dilute hydroxide. (b) Inversion of configuration in concentrated hydroxide. reactant, is trans (Figure 12.9a). A trigonal-bipyramidal intermediate with B in the trigonal plane leads to a mixture of trans and cis (Figure 12.9b). The incoming ligand can enter along any of the three sides of the triangle, resulting in two cis possibilities and one trans possibility. While only formation of !-cis isomers is shown in Figure 12.9, formation of ∆-cis reactant, is trans (Figure 12.9a). A trigonal-bipyramidal intermediate with B in the trigonal plane leads to a mixture of trans and cis (Figure 12.9b). The incoming ligand can enter along COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS any of the three sides of the triangle, resulting in two cis possibilities and one trans posSubstitution in trans complexes sibility. While only formation of !-cis isomers is shown in Figure 12.9, formation of ∆-cis Substitution Y for Xlikely in trans-[M(LL) (LL = a bidentate ligand) can proceed bylikely three isomers isofequally since ! and ∆ trigonal-bipyramidal intermediates are equally 2BX] dissociative pathways. in principle upon dissociation of X. From a purely statistical perspective, a racemic mixture of cis isomers should result. Dissociation to form a trigonal pyramid with B in an axial position (Figure 12.9c) allows two positions for attack by Y, both of which give cis products (the third side of the triangle is blocked by an LL ring). An intermediate with an axial B is less 1) If dissociation of X from the reactant leaves a square-pyramidal intermediate that adds likely than to onethe with an equatorial because an axial Bofrequires more rearrangement of the the new ligand vacant site, theB,result is retention configuration, and the product, ligands (a 90°is change like the reactant, trans. by one nitrogen and 30° changes by two others, in contrast to two 30° changes for the equatorial B) as well as a larger stretch for the LL ring in the equatorial plane. ation ochemical LL)2BX]. ntermediguration). dal interrmation own, a isomers X B -X +Y B Y B trans (a) Y B Y B ¶ tion chemical L)2BX]. ntermedi2) A uration). al intermation wn, a somers d" termeupon likely termediucts). changes for the equatorial B) as well as a larger stretch for the LL ring in the equatorial plane. COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS X Y -X +Y B B B trans trigonal-bipyramidal intermediate with B in the trigonal plane leads to a mixture of (a) trans and cis. Y B B -X X B Y B ¶ B Y ¶ B Y Y B B trans Y (b) • The incoming ligand can enter along any of the three sides of the triangle, resulting in two cis B Y possibilities and one trans possibility. B B X Y ¢ B B B Y ¶ omers " -X ermeCOORDINATION CHEMISTRY: REACTIONS AND MECHANISMS B B B B upon Y X ¶ Y ikely ermedi-3) Dissociation to form a trigonal pyramid with B in an axial position allows two positions for Y B B cts). attack by Y, both of which give cis products (the third side of the triangle is blocked by Y an LL ring). trans (b) B B Y B B B B X Y (c) Y ¢ Y ¶ An intermediate with an axial B is less likely than one with an equatorial B, because an axial B requires more rearrangement of the ligands (a 90° change by one nitrogen and 30° changes by two others, in contrast to two 30° changes for the equatorial B) as well as a larger stretch for the LL ring in the equatorial plane. 456 Chapter 12 | COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Coordination Chemistry IV: Reactions and Mechanisms FIGURE 12.10 Dissociation Mechanism and Stereochemical Changes for cis-[M(LL)2BX]. (a) Square-pyramidal intermediate (retention of configuration). (b) Trigonal-bipyramidal intermediate. (c) Unlikely trigonal bipyramidal intermediate. Substitution in cis complexes X Substitution in cis complexes can also proceed by three intermediates. ¢ B -X B +Y •The less likely trigonal bipyramid with an axial B, whether derived from a cis or a trans reactant, produces a racemic mixture of cis products. B ¢ (a) B • A square-pyramidal intermediate results in retention of configuration. • If dissociation of X forms a trigonal bipyramid with B in the trigonal plane, there are three possible locations for the addition of Y, all within this trigonal plane. Two of these result in cis products, with retention of configuration, and one in a trans product. Y X B B ¢ B Y B Y Y Y trans Y ¢ B B Y B ¢ (b) B X B Y B B B B ¢ Y (c) Y ¢ Y ¶ 100% cis isomer (Table 12.9), indicating a square-pyramidal transition state. Substitution COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Isomerization of Chelate Rings Pseudorotation The trigonal, or Bailar, twist, requires twisting the two opposite trigonal faces through a trigonal 12.6 Substitu prismatic transition state to the new structure ¢ ¶ ¢ (a) In tetragonal twists, one chelate ring is held stationary, while the other two are twisted to the new structure. Co Cl Cl (b) COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS 12.6 Substituti Substitution of Square-Planar Complexes | to 457 The tetragonal twist (below) has 12.6 a transition stateReactions with the stationary ring perpendicular those being twisted. FIGURE 12.11 Twist Mechanisms for Isomerization + of M(LL)3 and [Co(trien)Cl2] Complexes. (a) Trigonal twist: ¢ ¶ ¢ ¶ ¢ ¶ the front triangular face rotates (a) (b) with respect to the back trian(b) Another tetragonal twist requires twisting the two rings through a transition state with all gular face. (b) Tetragonal twist three rings parallel. with perpendicular rings: the Cl back ring remains stationary Co rotate as the front two rings Co Co clockwise. (c) ClTetragonal Cltwist Cl Cl Cl with parallel rings: the back ¢ ¶ a-¢ ring remains stationary as the ¶ a-¢ b-¶ (c) (d) Attempts to elucidate which specific twist is operative for a complex is an experimental challenge. front two rings rotate counter(d) clockwise. (d) [Co(trien)Cl2]+ α–β isomerization: the connected COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Substitution Reactions of Square-Planar Complexes The products of substitution reactions of square-planar complexes have the same configuration as the reactants, with replacement of the departing ligand by the new ligand. Kinetics and Stereochemistry of Square-Planar Substitutions Consider the generic reaction T⏤Pt⏤X + Y ⟺ T⏤Pt⏤Y + X where T is the ligand trans to the departing ligand X, and Y is the incoming ligand. Designate the plane of the molecule the xy plane and the Pt axis through T⏤Pt⏤X the x axis. Ignore the other two ligands for the moment. COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Reactions of square-planar complexes have significant associative character; they are classified as Ia. oordination Chemistry IV: Reactions and Mechanisms change nar ituted The Interchange Mechanism in Square-Planar Reactions. T Y L L T X L Y z L X T L L Y + •In (a), the incoming ligand Y approaches along the z axis. As it bonds to the Pt, the complex rearranges to approximate a trigonal bipyramid with Pt, T, X, and Y in the trigonal plane. As X leaves, Y moves into the plane of T, Pt, and the two L ligands. X y x (a) T L S L T X L S L X T +Y -X L Y L T S L Y L S T L L Y + S •The solvent-assisted mechanism (b) follows the same pattern but requires two associative steps. (b) replacing on the complexby through a similar 5-coordinate transition state, and then itself (a) DirectXsubstitution Y. (b) Solvent-assisted substitution. being replaced by Y. The second step of this mechanism is presumed to be faster than the first, and the concentration of solvent is large and unchanging (leading to pseudo first order conditions), so the overall rate law for this path is approximated as first order in complex. first, and the concentration of solvent unchanging (leadingto tobe pseudo being replaced by Y. The second stepisoflarge this and mechanism is presumed fasterfirst thanorder the first, and thesoconcentration of solvent large andisunchanging (leading to order pseudoinfirst order conditions), the overall rate law foristhis path approximated as first complex. COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS conditions), so the overall rate law for this path is approximated as first order in complex. 12.6.2 12.6.2 Evidence for Associative Evidence for a 5-coordinate intermediateReactions is strong, and the transition state sometimes may Evidence for a 5-coordinate intermediate is strong, and the transition state sometimes may Associative Reactions Evidence for Evidence Associativefor Reactions 23 Evidence for a 5-coordinate intermediate is strong, and the transition state sometimes even be 6-coordinate, with assistance from solvent. The highest energy transition may state even be 6-coordinate, with assistance from solvent. 23 evenbebeeither 6-coordinate, assistance from solvent. The energy transition state may during thewith formation of the intermediate or ashighest the exiting ligand dissociates maythe be either during the formation of the intermediate or as the exiting ligand dissociates from intermediate. Pt(II) from is aThis soft acid, so softreveals ligands react more readily with it. themechanism intermediate. an effect of the incoming ligand. Pt(II) is a soft acid, so soft mechanism reveals anit.effect of theof incoming ligand. is a on soft acid, soft The ligands orderThis of ligand reactivity depends on the otherPt(II) ligands the Pt.soon react more readily with Thesomewhat order ligand reactivity depends somewhat the ligands reacton more it. The order offor ligand reactivity depends somewhat on the The other rate constants for ligands thereadily Pt, butwith the rate constants other ligands on the Pt, but the rate constants for trans@PtL2Cl2 + Y h trans@PtL2ClY + Cltrans@PtL2Cl2 + Y h trans@PtL2ClY + Cl for different in methanol rank follows (Table 12.12): for different Y inYmethanol rank as as follows for different- Y in methanol rank as follows ( Table 12.12): PR3 7 CN - 7 SCN - 7 I - 7 Br - 7 N3 - 7 NO2- 7 py 7 NH3 ! Cl- 7 CH3OH PR3 7 CN 7 SCN 7 I 7 Br 7 N3 7 NO2 7 py 7 NH3 ! Cl 7 CH3OH Similar rankings are found for reactants with T ligands other than chloride. These Similar rankings are found for reactants with T ligands other than chloride. These8 rate constants vary over many orders of magnitude, with k(PPh )/k(CH OH) = 9 * 10 . 3 3 8 Similar are found reactants with T ligandswith other than chloride. raterankings constants vary overfor many orders of magnitude, k(PPh )/k(CH 3 3OH) = 9 * 10 . TABLE 12.12 Rate Constants and Nucleophilic Reactivity Parameters for TABLE 12.12 Rate Constants and Nucleophilic Reactivity Parameters for Entering Groups COORDINATION CHEMISTRY: REACTIONS AND MECHANISMS Because T and Y have similar positions in the transition state, these ligands have similar effects on the rate. This is called trans effect. Reactions of Square-Planar Complexes The leaving group X also has12.6 a Substitution significant influence on the rate.| 459 Rate 12.13 Constants for Leaving Groups TABLE Rate Constants for Leaving Groups [Pt(dien)X]+ + py h [Pt(dien)py]2+ + X− (Rate = (k1 + k2[py])[Pt(dien)X]+) X− NO3- k2 (M−1 s−1) •The order of X is nearly the reverse of that above, with hard ligands such as Cl−, NH3, NO3− leaving relatively quickly. very fast Cl - 5.3 * 10 - 3 Br - 3.5 * 10 - 3 I- 1.5 * 10 - 3 N3- 1.3 * 10 - 4 SCN - 4.8 * 10 - 5 NO2- 3.8 * 10 - 6 CN - 2.8 * 10 - 6 Rate constants calculated from data in F. Basolo, H. B. Gray, R. G. Pearson, J. Am. Chem. Soc., 1960, 82, 4200. •Soft ligands with considerable metal-to-ligand such as CN− and NO2−, leave relatively slowly. bonding •MTL bonding to X reduces the reactivity of Pt complexes significantly towards substitution of these l