In Fe(CO)5 the Fe-C bond posses both pi and sigma character . Why?

Carbon monoxide bonds to transition metals using "synergisticπ*back-bonding." The bonding has three components, giving rise to a partial triple bond. Asigma bondarises from overlap of nonbonding sp-hybridized electron pair on carbon with a blend of d-, s-, and p-orbitals on the metal. A pair of π bonds arises from overlap of filled d-orbitals on the metal with a pair of π-antibonding orbitals projecting from the carbon of the CO. The latter kind of binding requires that the metal have d-electrons, and that the metal is in a relatively low oxidation state (<+2) which makes the back donation process favorable. As electrons from the metal fill the π-antibonding orbital of CO, they weaken thecarbon-oxygen bondcompared with free carbon monoxide, while the metal-carbon bond is strengthened. Because of the multiple bond character of the M-CO linkage, the distance between the metal and carbon is relatively short, often < 1.8 Â, about 0.2 Â shorter than a metal-alkyl bond.