Mononuclear metal carbonyls:
The metal carbonyls which contain only one metal atom per molecule are called mononuclear metal carbonyls. Ex: [Ni(CO)4], [Fe(CO)5 ], [Cr(CO)6 ] , etc.
[Ni(CO)4]-Nickel tetracarbonyl
Preparations:
It can be prepared by passing carbon monoxide over nickel in the temperature of 25o C and 1 atm. pressure.
Ni + 4CO ---> Ni(CO)4
Properties:
It is a colorless liquid having boiling point 43o C and decomposition temperature in the range of 180–200o C. It is insoluble in water but dissolves in organic solvents.
Reactions:
Action of H2SO4: Nickel tetra carbonyl on treatment with Sulphuric acid undergoes oxidation to form Nickel sulphate
Ni(CO)4 + H2SO4 ---> NiSO4 +4CO(g) + H2(g)
Substitution: Nickel tetra carbonyl undergoes substitution reaction with electron donar ligands like triphenyl phosphine
Ni(CO)4 + 2 PPh3 ----> Ni(CO)2(PPh3)2 + 2CO(g)
Oxidation: Bromine oxidises Nickel tetra carbonyl in to Nickel Bromide.
Ni(CO)4 + Br2 ---> NiBr2 + 4CO(g)
Structure: Nickel tetracarbonyl has a tetrahedral geometry. It is also found to be diamagnetic.
The structure of Ni(CO)4 can be explained by considering sp3 hybridization of Ni atom. Since it is diamagnetic, all the ten electrons present in the valence shell of Ni atom (Ni = 3d8 4s2 ) get paired in 3d orbitals. Thus, the valence shell configuration of Ni atom in Ni(CO)4 molecule becomes 3d10 4s0 .
OC→Ni bond results by the overlap between the empty sp3 hybrid orbital on Ni atom and the HOMO on C atom in CO molecule.