Coordination Chemistry PDF

Chapter 7: Thermodynamic and Non redox kinetic factors in Coordination Chemistry Objectives: Mechanisms of Substitution reactions of octahedral and square planar complexes 1 Equilibrium constants of complex formation Example Larger K indicates bonding with incoming ligands more favorable than H 2O...

Chapter 7: Thermodynamic and Non redox kinetic factors in Coordination Chemistry Objectives: Mechanisms of Substitution reactions of octahedral and square planar complexes 1 Equilibrium constants of complex formation Example Larger K indicates bonding with incoming ligands more favorable than H 2O HSAB concept: Ag+ is soft and strongly bond with Brrelatively to F- and Cl2 Hard-Soft Acids and Bases 3 Factors that affect stability of complexes ∆G = -RTlnK = ∆H – TΔS allows calculation of free energy, entropy, and enthalpy of a reaction from stability constants Chelate effect 4 Reactions of coordination compounds Types of Reactions: Substitution Oxidation/Reduction Ligand Reactions … 5 Coordination Chemistry Reactions and Mechanisms Werner and Jorgenson discovered many of the basic reactions Experimentation over many years has yielded proposed mechanisms Mechanisms can’t be proven, only disproven – We can’t directly observe individual molecules react – Evidence either supports a mechanism or rules it out Goal: synthesize the predicted products by choosing the appropriate reaction conditions. 6 Substitution Reactions Inert and Labile Complexes (kinetic stability) Labile Complexes = those undergoing substitution with t½ < 1 minute c)Labile Metal ions = those with small or zero LFSE a)d1, d2, d7, d9, d10 b)High spin d4-d6 7 Kinetic Stability versus Thermodynamic Stability Stability (formation) constant = 2.0 x 1031 8 Substitution Reactions These reactions can produce colored products used to identify metal ions 9 Water exchange rate constants ( Table 12.1 ) vary widely as a function of the metal ion. Labile complexes Speeds of these reactions have been correlated to the electronic configuration of the starting complex Complexes such as [Cr(H2O)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. 11 Inert Compounds that react more slowly are called inert. Within this context, an inert compound does not resist ligand substitution; it is simply slower to react, with a higher activation energy for ligand substitution. These kinetic terms must be distinguished from the thermodynamic descriptions stable and unstable. Werner studied Co(III), Cr(III), Pt(II), and Pt(IV) compounds because they are inert and more readily characterized than labile compounds. 12 Inert octahedral complexes are generally those with high LFSE specifically those with d3 or low-spin d4 through d6 [Cr(H2O)6]3+ undergoes water exchange exceedingly slowly relative to the high-spin d4 [Cr(H2O)6]2+ [V(H2O)6]2+ reacts slower than [V(H2O)6]3+ 13 Chelate Effect The chelate effect causes polydentate complexes to be thermodynamically more stable than their monodentate counterparts. Substitution for a chelated ligand is generally a slower reaction than that for a similar monodentate ligand. Two factors: ΔH associated with removal of the first bound atom is larger than for a related monodentate ligand. kinetic barrier for subsequent reattachment is lower than for a related monodentate ligand since the former remains in close proximity to the metal center 14 Chelate Effect dissociation (1) is expected to be slower than a similar dissociation of ammonia, because the ethylenediamine ligand must bend and rotate to move the free amine away from the metal. 15 trans Effect In reactions of square-planar Pt(II) compounds, ligands trans to chloride are more easily replaced than those trans to ammonia chloride has a stronger trans effect than ammonia 16 trans Effect In (a) through (f), the first substitution can be at any position, with the second controlled by the trans effect. 17 Lability of Cl- In (g) and (h), both substitutions are controlled by the lability of chloride. 18 The trans-Effect in Pt(II) compounds – in square planar Pt(II) complexes Ligands trans to certain other ligands are easily substituted The controlling ligands: (-acceptor best) : Trans directors CO > CN- > olefins > H- > PR3 > NO2- ~ I- > SCN- > Br - > Cl - > NH3 ~ py > OH- > H2O 19 Contribution of Two factors: - Thermodynamic effect (trans influence): weakening of Pt-X bond and - A kinetic effect (trans effect): Stabilization of 5-coordinate transition sate. trans-Influence = ground state effect where the strong Pt—T sigma bond prevents the trans leaving group Pt—X bond from being strong. The weak Pt—X bond correspond to a high energy ground state The Ea required to get X to leave is small 20 21 The kinetic effect (trans effect) contribution − -acceptors remove e- density from Pt making association with Y more likely – This interaction from Pt—T lowers the energy of the 5-coord. intermediate – Ea is lowered and the Pt—X bond is more easily broken – Metal center will be more electrophilic for nucleophilic attack 22