Metal Carbonyls Notes PDF

Metal carbonyls 1. Polynuclear metal carbonyls: Mn2(CO)10; it’s preparations, structure and bonding  EAN Rule; according to which after ‘CO’ groups have donated a certain number of electron pairs to the zero valent metal atom through OC M σ – bonding,...

Metal carbonyls 1. Polynuclear metal carbonyls: Mn2(CO)10; it’s preparations, structure and bonding  EAN Rule; according to which after ‘CO’ groups have donated a certain number of electron pairs to the zero valent metal atom through OC M σ – bonding, the total number of electrons on the metal atom including those gained from CO molecules becomes equal to the number of the next inert gas. Example; Mn2(CO)10 EAN Rule; Electrons from two Mn atoms = 25 × 2 = 50 Electrons from 10 CO groups = 10 × 2 = 20 Electrons from one Mn – Mn bond = 1 × 2 = 2 ⸫ EAN of two Mn – atoms of Mn2(CO)10 = 72 Therefore; EAN of one Mn – atom in Mn2(CO)10 = 72/2 = 36 (Kr36) Alternative method: In Mn2(CO)10, since each Mn – atom is attached with 5 CO groups & with another Mn – atom (one Mn – Mn bond), EAN of ‘Mn’ in Mn2(CO)10 is given by: ⸫ EAN of Mn – atom = [Atomic number of Mn – atom] + [Electrons donated by five terminal carbonyl groups] + [Electrons donated by one Mn – Mn bond] ⸫ EAN of Mn – atom = 25 + (5 × 2) + (1 × 1) = 36 (Kr36)  18 – Electron Rule; According to this rule; in a stable metal carbonyl, total (ie; effective) number of electrons present in the valence shell (v. s.) of one metal atom is equal to 18. This number (ie; 18) is equal to EN/n; where ‘n’ is the number of metal (M) atoms in metal carbonyl, & ‘EN’ = Number of Electrons present in the valence shell of metal in carbonyl = Number of Electrons in the valence shell in the free State + (2 × Number of CO ligands) + (2 × Number of M – M bonds in the carbonyl)  Example; Mn2(CO)10 18 – Electron Rule; Mn2(CO)10; Mn(25) = 3d5 4s2 2 Mn = 2 × 7e- = 14e- 10 CO = 10 × 2e- = 20e- One Metal – Metal (Mn – Mn) bond = 1 × 2e- = 2e- ⸫ Total electrons on two Mn – atoms = 14e- + 20e- + 2e- = 36e- Therefore; electrons on one Mn – atom = 36e/2 = 18e- Structure & Bonding in Mn2(CO)10 O O C C C O O C 2.79 Ao O C Mn Mn C O O C C O C C O O The structure of dimanganese decacarbonyl consists of two manganese pentacarbonyl groups joined through a Mn – Mn bond distance of 2.79 A°. The formation of this metallic bond effectively adds one electron to each of the manganese atoms. Thus, manganese an element with odd atomic number from a binuclear carbonyl. Since the molecule does not have unpaired electrons, it is diamagnetic. The infra-red absoption spectral and diffraction studies have shown that this molecule contains no bridging carbonyl groups and hence the molecule has non – bridged structure as shown. It is evident from the above figure that each Mn – atom is linked with five terminal carbonyl groups ( :O ⃗¿ C → Mn ) & with the other Mn – atom by Mn – Mn sigma – bond. Thus, each Mn atom has  five :O ⃗¿ C → Mn coordinate sigma bonds and one Mn – Mn sigma – bond. The presence of Mn – Mn bond has also been supported by the diamagnetic nature of Mn 2(CO)10 molecule. It may be noted that each Mn – atom has an octahedral environment and two Mn(CO)5 units are joined solely by Mn – Mn bond. d2sp3 hybridization of each Mn – atom: Since it is evident from the non-bridged structure of dimanganese decacarbonyl that the number of sigma bonds by which each Mn – atom is linked with the terminal carbonyl groups and to the other Mn - atom is six, each Mn – atom has coordination number is equal to six and hence each Mn – atom is d2sp3 hybrised in Mn2(CO)10 molecule. Valence shell electronic configuration of Mn – atom in its free State is 3d5 4s2 4p0. When Mn – atom forms Mn2(CO)10 molecule both the electrons from 4s orbital get shifted to 3d orbitals and hence the configuration of Mn atom in Mn2(CO)10 becomes 3d7 4s0 4p0. In order to make d2sp3 hybridization possible, the distribution of seven electrons in 3d orbitals takes place in such a way that 3dz2 orbital becomes vacant and hence the configuration of Mn atom in Mn 2(CO)10 molecule becomes 3d2xy 3d2yz 3d2zx 3d1x2-y2 3d0z2. Now 3dx2-y2, 3dz2, 4s, 4px, 4py & 4pz orbitals (six orbitals) combine together to form six d2sp3 hybrid orbitals. 3dxy, 3dyz and 3dzx orbitals (each is completely filled) remain unhybridized. One of the six d2sp3 hybrid orbital is singly-filled while the remaining five are vacant. Mn (Z=25) Electronic configuration: 1s2 2s2 2p6 3s2 3p6 3d5 4s2 3d 4s 4p (a)Mn – atom in free state (3d5 4s2 4p0) Six d2sp3 hybrid orbitals (b) One Mn – atom in z2 Mn2(CO)10 molecule: (3d7 4s0 4p0) CO CO CO CO CO Mn – Mn σ −bond CO CO CO CO CO (c) Other Mn – atom in Mn2(CO)10 molecule: (3d7 4s0 4p0) z2 Six d2sp3 hybrid orbitals Note: d2sp3 hybridisation of Mn – atom in Mn 2(CO)10 molecule which has Octahedral geometry. Each pair of crosses (coloured arrows) indicates electron pair donated by C – atom of terminal carbonyl group. Thus, the valence shell configuration of Mn atom in d 2sp3 hybridized state in Mn2(CO)10 molecule can be written as shown above. The singly filled d2sp3 hybrid orbitals of one Mn atom overlaps with the singly-filled d2sp3 hybrid orbitals of another Mn atom and gives Mn – Mn sigma bond. Thus Mn – Mn is formed by d2sp3 (Mn) – d2sp3 (Mn) overlap. ‘sp’ hybrid orbital of carbon atom of each CO group donates its lone pairs of electrons to vanact d2sp3 hybrid orbital on Mn atom and produces :O ⃗¿ C → Mn coordinate sigma bond. Thus; this bond results by [sp (C) – d2sp3 – (Mn)] overlap. Thus, we see that Mn – atom is linked with five CO groups by five :O ⃗¿ C → Mn bonds and with one Mn – atom get paired and hence Mn 2(CO)10 shows diamagnetic character the pairing of electrons occurs due to the formation of Mn – Mn bond. Preparations of manganese decacarbonyl i. It can be prepared by carbonylation of manganese iodide with carbon monoxide using mangnesium as a reducing agent. ii. 2MnI2 + 10CO + 2Mg 250C, 210 atm Mn2(CO)10 + 2MgI2 OR Mn2(CO)10 is obtained when MnI2 (prepared by special method) is reduced at high pressure of CO by Mg in diethyl ether. The yield of Mn2(CO)10 is only 1%. Mn + 2CuI 5000C MnI2 + 2Cu CO, 250C, 210 atm/16 hours (MnI2 + 2Cu) + Mg Mn2(CO)10 + 2MgI2 + CuI2 iii. Mn2(CO)10 has also been prepared in better yield by the reaction of Mn(CH3 – COO)2 with triethyl aluminium, (C2H5)3Al under CO at a pressure of 20 atmospheres at 100 oC for 5 hours. 2Mn(CH3COO)2 + (C2H5)3Al CO, 1000C, 20 atm/ 5 hours Mn2(CO)10 The yield of carbonyl by this method is 53 – 60% iv. Carbonylation of MnCl2 in presence of benzophenone ketyl like (C 6H5)2CONa at 165oC gives 35% yield Mn2(CO)10.  MnCl2 + 2(C6H5)2CONa Mn[CO(C6H5)2]2 CO, 1650C, 140 atm Mn[CO(C6H5)2]2 Mn2(CO)10 + (C6H5)2CO v. More recently 48% yield of Mn2(CO)10 has been obtained by the carbonylation of methyl cyclopentadienyl manganese dicarbonyl, (CH3 – C5H4)Mn(CO)3 in presence of Na at 50 atmospheric pressure of CO and 125oC for 8 hours. (CH3C5H4)Mn(CO)3 + Na CO, 1250C, 50 atm/ 5 hours Mn2(CO)10 Properties: i) It is a golden yellow crystalline substance. It has a melting point of 155oC and sublimes in vacuum. ii) It is slowly oxidized in air, especially in solution. In other words, it is oxidized by trace of amount of oxygen in solution; hence the solution must be stored in inert atmosphere. iii) It is soluble in organic solvents. iv) Action of halogen: Halogenation of dimanganese decacarbonyl proceeds with breaking of Mn – Mn bond and formation of carbonyl-halides (Mn(CO)5X) as shown below: Mn2(CO)10 + Br2(l) 40oC 2Mn(CO)5Br Mn2(CO)10 + I2 2Mn(CO)5I The order of activity of halogen is as: I > Br > Cl v) Action of Li and Na Mn – Mn bond is broken by lithium in presence of tetra hydro furan (THF) and by Na in presence of liq. NH3 and Li+ [Mn(CO)5]- is formed. Mn2(CO)10 + 2Li THF 2Li+[Mn(CO)5]- Mn2(CO)10 + 2Na liq. NH3 2Na+[Mn(CO)5]- vi) Reduction When Mn2(CO)10 is reduced by hydrogen under 200 atmospheric pressure at a temperature of 200oC, carbonyl hydride is formed. o Mn2(CO)10 + H2 200 atm., 200 C 2HMn(CO)5 vii) Action of phosphines, arsines & stilbines With these materials Mn2(CO)10 gives monomeric paramagnetic compounds of the type, Mn(CO)4(PR3). viii) Diamagnetic nature Mn2(CO)10 is a diamagnetic substance. Diamagnetic character confirms the facts that all the electrons in Mn2(CO)10 are paired and Mn – Mn bond is also present in it.  Metal carbonyls 1. Polynuclear metal carbonyls: Co2(CO)8; it’s preparations, structure and bonding in solution & Solid state  EAN Rule; according to which after ‘CO’ groups have donated a certain number of electron pairs to the zero valent metal atom through OC M σ – bonding, the total number of electrons on the metal atom including those gained from CO molecules becomes equal to the number of the next inert gas. Example; Co2(CO)8 EAN Rule; Electrons from two Co atoms = 27 × 2 = 54 Electrons from 8 CO groups = 8 × 2 = 16 Electrons from one Co – Co bond = 1 × 2 = 2 ⸫ EAN of two Co – atoms of Co2(CO)8 = 72 Therefore; EAN of one Mn – atom in Mn2(CO)10 = 72/2 = 36 (Kr36) Alternative method: a) Co2(CO)8 in solution – In solution Co2(CO)8 has non – bridged structure in which each Co – atom is linked with four terminal CO groups & with another Co – atom (one Co – Co bond). Thus, EAN of Co – atom is given by: ⸫ EAN of Co – atom = [Atomic number of Co – atom] + [Electrons donated by four terminal carbonyl groups] + [Electrons donated by one Co – Co bond] ⸫ EAN of Co – atom = 27 + (4 × 2) + (1 × 1) = 36 (Kr36) b) Co2(CO)8 in solid state – In solid state Co2(CO)8 has bridged structure in which each Co – atom is linked with three terminal CO groups, two bridging CO groups & one Co – atom (one Co – Co bond). Thus, EAN of Co – atom is given by: ⸫ EAN of Co – atom = [Atomic number of Co – atom] + [Electrons donated by three terminal carbonyl groups] + [Electrons donated by two bridging carbonyl groups] + [Electrons donated by one Co – Co bond] ⸫ EAN of Co – atom = 27 + (3 × 2) +(2 × 1) + (1 × 1) = 36 (Kr36)  18 – Electron Rule; According to this rule; in a stable metal carbonyl, total (ie; effective) number of electrons present in the valence shell (v. s.) of one metal atom is equal to 18. This number (ie; 18) is equal to EN/n; where ‘n’ is the number of metal (M) atoms in metal carbonyl, & ‘EN’ = Number of Electrons present in the valence shell of metal in carbonyl = Number of Electrons in the valence shell in the free State + (2 × Number of CO ligands) + (2 × Number of M – M bonds in the carbonyl) Example; Co2(CO)8 18 – Electron Rule; Co2(CO)8; Co(27) = 3d7 4s2 2 Co = 2 × 9e- = 18e- 8 CO = 2 × 8e- = 20e- One Metal – Metal (Co – Co) bond = 1 × 2e- = 2e- ⸫ Total electrons on two Co – atoms = 18e- + 16e- + 2e- = 36e- Therefore; electrons on one CO – atom = 36e/2 = 18e- Structure & Bonding in Co2(CO)8 a) Non – bridged structure of Dicobalt Octacarbonyl molecule, CO2(CO)8 molecule in solution The infra-red study of the solution of CO2(CO)8 has no bridging carbonyl groups ie; CO2(CO)8 in solution has non – brigded structure. O O C C C O O C 2.52 Ao O C Co Co C O O C C O C C O O In this structure each CO – atom is linked with four terminal CO group (OC Co) & with other Co – atom by a Co – Co σ −bond. Thus, each CO – atom has four OC Co coordinate σ −bonds and one Co – Co σ −bond. The presence of Co – Co bond has also been supported by the diamagnetic nature of CO2(CO)8 molecule. Co – Co bond length is 2.52 Ao has been determined by electron diffraction study. dsp3 hybridization of each CO – atom Since it is evident from the non-bridged structure CO2(CO)8 molecule, the number of σ −bonds by which each CO – atom is linked with terminal carbonyl groups and to the other Co – atom is five, each CO – atom is dsp3 hybridized in CO2(CO)8 molecule. Valence shell electronic configuration of Co – atom in its free state is 3d7 4s2 4p0. When Co – atom forms CO2(CO)8 molecule, both electrons from 4s orbital get shifted to 3d orbitals and hence the valence shell configuration of Co – atom in CO2(CO)8 becomes 3d9 4s0 4p0. These two configurations have been shown at (a) and (b) respectively. Now one 3d – orbital ie., 3dz2 (which is singly – filled with electron) one 4s orbital (vacant) and all the three 4p – orbitals (vacant) combine together and give five dsp3 hybrid orbitals.  Thus, we can see that four 3d orbitals (each of which is completely filled) remain unhybridized. One of the five dsp3 hybrid orbitals has one electron (singly-filled) while the remaining four are vacant. dsp3 hybridized state of Co – atom has shown at (c). Co (Z=27) Electronic configuration: 1s2 2s2 2p6 3s2 3p6 3d7 4s2 3d 4s 4p (a)Co – atom in free state (3d7 4s2 4p0) (b) One Co – atom in dsp3 five dsp3 hybrid orbitals hybridised Co2(CO)8 2 z molecule: (3d9 4s0 4p0) CO CO CO CO Co – Co σ −bond CO CO CO CO (c) Other Co – atom in Co2(CO)8 molecule: (3d9 4s0 4p0) z2 five dsp3 hybrid orbitals Note: dsp3 hybridisation of Co – atom in Co2(CO)8 molecule in solution. Each pair of coloured arrows indicates electron pair donated by C – atom of CO molecule (ligand). The singly-filled dsp3 hybrid orbitals of one Co – atom overlaps with the singly-filled dsp3 hybrid orbital of the other Co – atom and produces Co – Co σ −bond. This Co – Co σ −bond is produced by [dsp3 (Co) – dsp3 (Co)] overlap. sp – hybrid orbital on C – atom of each terminal Co group donates its lone pair of electrons to the vacant dsp3 hybrid orbital on Co – atom and gives OC Co coordinate σ −bond , ie., this bond results by [sp(C) – dsp3(Co)] overlap. Thus, we can see that each CO – atom is linked with four Co groups by OC Co bonds and with one Co – atom by Co – Co bond. It may be seen here that all the electrons of both Co – atoms become paired and hence CO2(CO)8 shows diamagnetic character. The pairing of electrons takes place due to the formation of Co – Co bond. b) Bridged structure of Dicobalt Octacarbonyl Molecule, CO2(CO)8 in solid state  Infra – red study of CO2(CO)8 molecule in the solid state has indicated that this molecule has bridged structure as shown below: O C C O O C 2.52 Ao O C Co Co C O O C C O C O d2sp3 hybridization of each Co – atom. Since it is evident from the above figure that the number of σ −bonds by which each Co – atom is linked with three terminal carbonyl groups, two bridging carbonyl groups and to the other Co – atom is six, each Co – atom has a coordination number equal to six and hence each Co – atom is d2sp3 hybrized in CO2(CO)8 molecule. Valence shell electronic configuration of Co – atom in the free state is 3d7 4s2 4p0. When Co – atom forms CO2(CO)8 molecule, one of the two electrons of 4s orbital is shifted to 3d orbitals and hence the valence shell configuration of Co – atom becomes 3d8 4s1 4p0. These two configurations have been shown at (a) and (b). Now the two 3d orbitals ie., 3dx2-y2 and 3dz2 (both are singly-filled), one 4s orbital (singly filled) and all three 4p orbitals (all are empty) combine togather and produce six d2sp3 hybrid orbitals. Thus, we can see that three 3d orbitals (each of which is completely filled) remain unhybridized. Three hybrid orbitals are having one electron each while the remaining three hybrid orbital are vacant. Thus, the valence shell configuration of Co – atom in d2sp3 hybrized state can be depicted as shown at (c). Co (Z=27) Electronic configuration: 1s2 2s2 2p6 3s2 3p6 3d7 4s2 3d 4s 4p a) Co – atom in free state (3d7 4s2 4p0) Six d2sp3 hybrid orbitals b) One Co – atom in dsp3 hybridised Co2(CO)8 z2 molecule: (3d8 4s1 4p0) Co – Co CO CO CO σ −bond CO CO Two bridging CO groups CO CO CO c) Other Co – atom in Co2(CO)8 molecule: (3d8 4s1 4p0) z2 six d2sp3 hybrid orbitals Note: dsp3 hybridisation of Co – atom in Co2(CO)8 molecule in solid state. Each pair of coloured arrows indicates electron pair donated by C – atom of terminal carbonyl group; CO while the dot ( ) represents an electron donated by C – atom of bridging carbonyl group. Now one singly filled d2sp3 hybrid orbital of both Co – atom overlaps with both the singly-filled sp2 hybrid orbitals of C – atom of one bridging carbonyl group and form two Co – C bond in (Co)2 −C=Ö : CO O CO C Co OC CO Co C OC O OC Unusual overlap of two singly – filled d2sp3 hybrid orbitals on two Co – atoms to form Co2(CO)8 molecule in solid state. In a similar way other two Co – C bonds are also produced by one bridging carbonyl group. Thus, we see that four Co – C bonds are obtained. Here C – atom belongs to bridging carbonyl group. It is easy to understand that each of these four bonds results by [d2sp3(Co) – sp2(C)] overlap, third singly filled d2sp3 hybrid orbital of the other Co – atom and produces Co – Co bond. Thus, we can say that Co – Co bonds results by [d2sp3(Co) – d2sp3(Co)] overlap. each of the remaining three d2sp3 hybrid orbital (each is vacant) of each Co – atom accepts one lone pair of electrons donated by the sp hybrid orbital on C – atom of terminal carbonyl group and forms :O ⃗¿ C →Co coordinate bond. Thus, six :O ⃗¿ C →Co coordinate bonds are obtained. Obviously, this bond is obtained by [sp(C) – d2sp3(Co)] overlap. the above discussion shows that each Co – atom is linked with three terminal carbonyl groups by :O ⃗¿ C →Co bonds. It may be noted that all the electrons in both Co – atoms become paired and hence Co2(CO)8 molecule shows diamagnetic character. The pairing of electrons occurs due to the formation of Co – Co bond. The formation of various bonds in Co2(CO)8 has shown in the figure. Non – bridged structure of Co2(CO)8 found in its solution is reported to exist in equilibrium with the bridged structure in solid state. Co – Co bond which is obtained by the overlap of two singly – filled d 2sp3 hybrid orbitals on two Co – atoms is bent because of unusual overlap of two d2sp3 hybrid orbitals. Preparations of Dicobalt Octacarbonyl i. It can be prepared by the reaction between CO and reduced metallic cobalt at 200 oC and 250 atm. ii. 2200C, 250 atm 2Co + 8CO Co2(CO)8 ii. It can be also synthesized by the reaction of dry CO at 200 oC and 200 atm in presence of metallic copper on certain binary compounds of cobalt such as CoS and CoX2. Here Cu forms Cu2S or CuX. iii. 2200C, 250 atm 2CoS + 8CO + 4Cu Co2(CO)8 + 2Cu2S iv. 2200C, 250 atm 2CoX2 + 8CO + 4Cu Co2(CO)8 + 4Cu2X iii. It can be prepared by the action of CO and H2 on CoCO3 under high pressure and high temperatures. v. 120-2200C, 250-300 atm 2CoCO3 + 2H2 + 8CO Co2(CO)8 + 2CO2 + 2H2O  iv. It can be prepared by the action of an acid on a solution of Co(CO) 4. Hydrogen is evolved and Co2(CO)8 is left behind. 2H+[Co(CO)4]- Co2(CO)8 + H2 v. It can be prepared by thermal decomposition of Co(CO)4H. Properties: i) Co2(CO)8 is an orange brown crystalline substance having melting point of 51oC. It is soluble in alcohol, ether and carbon tetrachloride. The carbonyl is air sensitive both in the solid and solution state. ii) Action of heat: It is thermally decomposed at 50oC in an inert atmosphere to give tetra cobalt decacarbonyl, Co4(CO)12 2Co2(CO)8 50oC Co4(CO)12 + 4CO iii) Action of air: On exposure to air, dicobalt octahedral is converted into deep violet basic carbonate of cobalt. iv) Reduction a) It is reduced to cobalt hydride, H+[Co(CO)4]- by H2 at 165oC and at 120 atmospheric pressure. Co2(CO)8 + H2 120 atm., 165oC 2H+[Co(CO)4]- Co2(CO)8 is also reduced by Na metal in liq. NH3 below -75oC or tetrahydrofuran (THF). v) Disproportionation reactions: There are two types of reactions in case of Co2(CO)8. a) Strong bases having nitrogen or oxygen donor atoms cause proportionation into Co(+2) and CO(-1). For example; 2Co2(CO)8 + 12NH3 2[Co(NH3)6][Co(CO)4]2 + 8CO b) With isocynides, phosphines, arsines and stilbines; however the disproportionation reaction gives penta-coordinate Cobalt(I) cation.  Co2(CO)8 + 5CNR [Co(CNR)5][Co(CO)4] + 4CO vi) Reaction with NO: Co2(CO)8 also reacts with NO at 40oC and forms Co(CO)3NO. Co2(CO)8 + 2NO Co(CO)3(NO) + 2CO In Co(CO)3(NO), NO group appears to be more firmly held with Co – metal atom than CO groups. This is confirmed by the fact that CO groups can be replaced by the amines very easily. Eg; the action of phen on Co(CO) 3(NO) gives [Co(CO)(phen)(NO)]. Co(CO)3(NO) + phen [Co(CO)(phen)(NO)] + 2CO vii) Reaction with halogens: The halogens decompose Co2(CO)8 according to the following reaction: Co2(CO)8 + 2X2 2CoX2 + 8CO