Structure of Metal Carbonyls
(a) tetracarbonyl nickel (0) is tetrahedral
(b) pentacarbonyl iron (0) is trigonal bipyramidal
(c) hexacarbonyl chromium (0) is octahedral
(a) decacarbonyl manganese (0) is made up of two square pyramidal Mn(CO)5 units joined by Mn-Mn bond.
(b) Octacarbonyl dicobalt (0) has a Co-Co bond bridged by two Co-Co groups.
Preparation of Metal Carbonyls
Metal carbonyls are prepared by the direct interaction of the finely divided metal and carbon monoxide:
Ni + 4CO → Ni(CO)4
(ii) Metal carbonyls are prepared by a suitable metal salts and carbon monoxide in the presence of reducing agent. The metal salts are reduced by CO.
General characteristics of Metal Carbonyls
(2) The mononuclear carbonyls are volatile and toxic.
(3) With the exception of Fe2(CO)9, enneacarbonyldiron (0), carbonyls are soluble in hydrocarbon solvents.
(4) Mononuclear carbonyls are either colourless or light coloured.
Fe(CO)5 is light straw-coloured liquid.
Polynuclear carbonyls are more deeply coloured.
Fe3(CO)12 dodecacarbonyltriiron (0) is a deep grass green solid.
(i) the metal centre of the carbonyl and
(ii) the CO ligands.
Bonding in Metal Carbonyls
(2) When CO acts as a ligand, this orbital (σ*2s) serves as a weak donor to the metal atom and forms a M ← C bond.
(3) The lowest unoccupied MOs of CO are the π* MOs. These vacant orbitals overlap with metal d-orbitals having π-symmetry. This results in a bond from filled d-orbitals on the metal into vacant orbitals on CO ligands. This is called back bond.
The bonding in metal carbonyls is described by the following steps:
(1) There is a donation of lone pair of electrons of carbon (of CO) into the suitable empty orbital of the metal atom. This is a dative overlap and forms a sigma M← C bond.
(2) There is a π-overlap involving donation of electrons from filled metal d-orbitals into vacant antibonding π* molecular orbitals of CO. This results into the formation of M→C π bond. This is also called back donation or back bonding.
The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal.