Explain homoleptic carbonyl compounds.

(N/A) Homoleptic carbonyls are those metal carbonyls in which only carbonyl $(CO)$ ligands are attached to the metal atom.
These compounds are formed by most transition metals. These carbonyls have simple,well-defined structures. $[Ni(CO)_4]$,$[Fe(CO)_5]$,and $[Cr(CO)_6]$ have tetrahedral,trigonal bipyramidal,and octahedral geometries,respectively.
Decacarbonyldimanganese $(0)$,$[Mn_2(CO)_{10}]$,is formed by the linkage of two square pyramidal $[Mn(CO)_5]$ units through a $Mn-Mn$ bond. Octacarbonyldicobalt $(0)$,$[Co_2(CO)_8]$,has a $Co-Co$ bond bridged by two $CO$ groups.
In metal carbonyls,the metal-carbon bond possesses both $\sigma$ and $\pi$ character. The $M-C$ $\sigma$ bond is formed by the donation of lone pairs of electrons from the carbonyl carbon into the vacant orbital of the metal.
The $M-C$ $\pi$ bond is formed by the donation of a pair of electrons from a filled $d$-orbital of the metal into the vacant antibonding $\pi^*$ orbital of $CO$. The metal-to-ligand bonding creates a synergic effect,which strengthens the bond between the metal and $CO$.