CHAPTER 11 Metal-II Complexes: •!• Metal Carbonyls: Structure and Bonding The bonding in coordination compounds is usually visualized as the donation of ligand electron pair to the metal centre only. However, there are some ligands which not only have filled atomic orbitals (donor orbitals) but also have some empty orbitals (acceptor orbitals) of appropriate symmetry and energy to accept electron density from central metal atom or ion. This interaction is called n:-backbonding or n:-backdonation; and is generally shown by CO, NO, PR3 and alkene-alkyne type li gands. Two of the most common examples where n:-backbonding occurs include Ni(C0)4 and Zeise's salt. Furthermore, metal carbonyls are one of the most widely studied type of metal-n: complexes, that can simply be defined as the coordination compounds of transition metals with carbon monoxide as a ligand. Metal carbonyls are very useful in synthetic organic chemistry and in homogeneous catalysis, like the process of hydroformylation. In the Mond process, nickel carbonyl is used to produce pure nickel. In organometallic chemistry, metal carbonyls act as precursors for the synthesis of many organometalic compounds. Metal carbonyls are toxic by inhalation, skin contact, or ingestion, in partially due of their ability to attach to iron of hemoglobin to give carboxyhemoglobin, which inhibits the binding of dioxygen. Metal carbonyls can be classified on the basis of number of metal centres; mononuclear carbonyls have only one metal atom or ion such as Fe(CO)s, while polynuclear carbonyls contain more than one metal centre like homonuclear Fe2(C0)9 and heteronuclear MnRe(C0)10. One more categorization basis of metal carbonyls is the bonding profile of carbonyl li gand; non-bridging carbonyls and bridging carbonyls. Non-bridging carbonyls may or may not contain metal-metal bond. For instance, Ru(CO)s and Mn2(C0)10 both have only terminal carbonyl groups but Mn2(C0)1o has one metal-metal bond also. On the other hand, bridging metal carbonyls like Fe3(C0)12, in addition to terminal CO groups, do have CO groups bridged to more than one metal centres. ~ General Methods ofPreparation 1. By direct reaction: Some of the mononuclear carbonyls can be prepared by the direct reaction of carbon monoxide with metal powder. 2s 0 c Ni + 4CO -----+ Ni(C0)4 1 atm 200°c Fe+ SCO ---------+ Fe(C0)5 200 atm 1so 0 c 2Co + 8CO ---------+ Co 2 (C0)8 35 atm DALAL D INSTITUTE CHAPTER 11 Metal-n Complexes: 427 2. By reduction: One of the most widely used method to synthesize metal carbonyls is the reduction of corresponding metal salts in the presence of carbon monoxide. AICl 3 CrCl3 +Al + 6CO ---------+ Cr(C0)6 + AICl 3 benzene diglyme, 100°c VCl 3 + 4Na + 6CO ---------+ [(diglyme) 2 Na] [V(C0) 6] + 3NaCI high pressure 200°c 2COl 2 + 4Cu + sco-----~ Co 2 (C0)8 + 4Cul 200 atm 120 - 200°c 2CoC0 3 + 2H 2 + SCO --------+ Co2 (C0)8 + 2H 2 0 + 2C02 250 - 300 atm 250°C 350 atm In the last reaction, carbon monoxide is the reducing agent on its own. 3. From mononuclear carbonyls: Iron pentacarbonyl is sensitive to light and air and can be used to synthesize Fe2(C0)9 by direct photolysis. hv 2Fe(C0)5 Fe2 (C0)9 +CO Similarly hv 20s(C0)5 Os 2 (C0)9 +CO hv 2Ru(C0)5 Ru 2 (C0)9 + CO 4. From iron pentacarbonyl: Carbon monoxide ligands in Fe(CO)s are labile and therefore can be used synthesize other metal carbonyls. 110°c MoC1 6 + 3Fe(C0)5 -------+ Mo(C0)6 + 3FeCl2 + 9CO ether 110°c WCl 6 + 3Fe(C0)5 ---------+ W(C0)6 + 3FeCl2 + 9CO ether 5. From metathesis reaction: Mixed-metal carbonyls can successfully be prepared via metathesis reaction route as: KCo(C0) 4 + [Ru(C0)3 Cl2 ]z ------+ 2RuCo2 (C0)11 + 4KCI DALAL D INSTITUTE 428 A Textbook of Inorganic Chemistry - Volume I ~ Structures ofMetal Carbonyls The structure of metal carbonyls can mainly be classified into three categories; first as the mononuclear systems that contain only one metal atom, the second one as binuclear systems that may or may not contain bridging carbonyls, and the last one as the polynuclear systems which contain more than two metal centres with all terminal, all bridging, or a mixture of two types of carbonyl groups. 1. Mononuclear metal carbonyls: The structure of mononuclear metal carbonyls is pretty simple and easy to visualize. This is definitely due to the presence of only one metal centre. The general examples of mononuclear metal carbonyls include tetrahedral Ni(C0)4 and Pd(C0)4; the trigonal bipyramidal case ofFe(CO)s, Ru(CO)s and Os(CO)s; and the octahedral geometries of V(C0)6, Cr(C0)6, Mo(CO)s and W(C0)6. co co I \\\\111CO oc ,,,,, I 11\\\CO oc--M·"' '''• .. M ..,, ,\ 1~co ocfll"""' I ""co co co M(C0)4 M(C0)5 M(C0)6 M - Ni, Pd M - l't:, Ru, Os M - V, Cr, Mo, W Figure 1. The structures of some mononuclear metal carbonyls. 2. Binuclear metal carbonyls: The structure of the binuclear metal carbonyls comprises of two metal centres and involve either metal-metal bonds or bridging CO groups, or both. For example, the Co2(C0)8 is known to exist in two isomers. The first one of has a D 3d symmetry with one metal-metal bond with zero bridging carbonyl; the second one is of C2v symmetry and has two bridging CO ligands along with one metal-metal bond. The structure of Fe2(C0)9 exist with D Jh symmetry, and contains three bridging CO ligands and six terminal CO groups attached. Furthermore, M2C010 (M =Mn, Tc, Re) exists with D 4d symmehy with one metal-metal bond and four CO ligands attached to each of the metal centre. c0 ~o ~~ co oc ~ ~ ~o~ I \ ! oc"' .f; c ~.. /co oc--co----c0 --co .Co Co '!<.,,, •••, \'•,,,,, 0 I \ I oc~ co oc t~ o co oc co Co2(C0)8 Figure 2. continued on next page ... DALAL D INSTITUTE CHAPTER 11 Metal-n Complexes: 429 c0 oc co oc ~~ co lo I co ~:::-o~ / \.:~ ~! ..~ c~.. OC --M -----M--CO .Fe Fe. oc''/ ""c/ \'11co I\ /I oc 0 co oc co oc co M 2(C0)10 M - Mn, Tc, Re Figure 2. The structures of some binuclear metal carbonyls. 3. Polynuclear metal carbonyls: The structures of the polynuclear metal carbonyls comprises of three or more metal centres and involve all bridging, all terminal, or a mixture of two types of CO groups. For example, the Ru3(C0)12 cluster has D3h symmetry, comprises of an equilateral triangle of Ru centres, each of which has two axial and two equatorial CO ligands. Os3(C0)12 has the same structure, whereas Fe3(C0)12 is different, with two bridging CO ligands, resulting in C2v symmetry. M4C012 (M =Co, Rh) is consisted of a tetrahedral M4 core, but the molecular symmetry is C3v· Three carbonyl ligands are bridging ligands and nine are terminal. However, lr4(C0)12 has perfect Td symmetry with no bridging CO ligands groups. The Rh4 and Jr4 clusters are more thermally robust than that of the Co4 compound, reflecting the usual trend in the strengths of metal-metal bond for second and third row metals vs those for the first row metals. Furthermore, [Re4(C0)16]2- has D2h symmetry with no bridging carbonyl. Furthermore, the structure of Os4(C0)16, Os4(C0)1s and Os4(C0)14 are somewhat more complex because of non-rigidity. The tetranuclear Os4(C0)16 is analogues to the cyclobutane with a puckered structure. The X-ray diffraction analysis of Os4(C0)14 unveiled an irregular tetrahedral Os4 skeleton with four weakly semi-bridging CO groups and four different Os-Os bond lengths. The experimental structure of Os4(C0)1s was determined to have a planar butterfly-like geometry consisting of two triangles sharing an edge. The hexanuclear M6(C0)16 (M = Rh, Co) exists with an octahedral core with alternate faces participating in the bridging; i.e. with four triply bridged and twelve terminal carbonyls. 0 ;::,c~ oc ~tg~/co oc'''''7 _.. Fe~ l/Fe\'''''''''''co /~e'-. \ oc oc ~ co co co M3(C0)12 M = Os, Ru Fe3(C0)12 Figure 3. continued on next page ... DALAL D INSTITUTE 430 A Textbook of Inorganic Chemistry - Volume I co oc, I ,......co M oc ' / /';___ __ .-8 .. / co 'r.,1- -• -• -• - ~•.•• M oc /'.~Le\ ,' :"' co OC------ M -------CO I\ oc co M4(C0)12 M - Co,Rh co oc co oc'%.. / co \r _os ~OC1111,,. I ,,,,,,,co -----=:;;:::='Os ""'CO oc"'j """ ~·os·?--- ~ \ oc I I co Os CO oc,.- I ~co co co OC11,,,, I ,,,,,co co ''•·. - ···'' co ,,,,,,,,,,,, 01s ,,,,,,,,,,,,,,, I ~co I ,,,,,,\ ,,,,,,,, ....~ oc -T~ Co r /f--co co ~otl"' co oc,,,, I ' co co Figure 3. continued on next page ... DALAL D INSTITUTE CHAPTER 11 Metal-n Complexes: 431 oc~ /co OC' -- ,.M\,'· ·- , , ··~:c;o '" , ' I', ,' I f \. , I ' ' ,. \ I \. , t I '\ -., " 1 OC ··~" , OC- -M- -:..--c ',, ""M :' ~"-:-',/ / ---.....__~·/ '\ \ _..,,..co / .. ~ ". --- ~1 ....... oc ""'·~r-:-,-::-ko ~- - /':' ....... co DALAL INSTITUTI~/.." ...~·/::c~· Main Market, Sector-14, Rohtak, Haryana /M......___ www.dalalinstitute.com, +91-9802825820 oc co NET.JRF, Ill-GATE, Ill.JAM & M.Sc Entrance (Chemistry) tvii;(CO) i6 M = Co,Rh Figure 3. The structures of some polynuclear metal carbonyls. ~ Bonding in Metal Carbonyls In order to rationalize the nature of the bonding between metal centre and the carbonyl ligand, we must understand the bonding within the carbonyl ligand itself first. It is a quite well known fact that CO group acts as a good sigma donor as well as good n-acceptor ligand.