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Discuss briefly the nature in metal carbonyls. |
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Answer» Solution :`(i)` In metal carbonyls, the bond between metal atom and the carbonyl ligand consists of two COMPONENTS. `(ii)` The first component is an electron pair donation from the CARBON atom of carbonyl ligand into a vacant `d`-orbital of central metal atom. This electron pair donation forms `M overset(sigma bond)(larr) CO` sigma bond. `(iii)` This sigma bond formation INCREASES the electron density in metal `d`-orbital and makes the metal electron rich. `(iv)` In order to compensate for this increased electron density, a filled metal `d`-orbital interacts with the empty `pi^(*)` orbital on the carbonyl ligand and transfers the added electron density back to the ligand. This second component is called `pi-`back bonding. `(v)` Thus in metal carbonyls, electron density MOVES from ligand to metal through sigma bonding and from metal to ligand through pi bonding this synergic effect accounts for strong `M larr CO` bond in metal carbonyls. This phenomenon is shown diagrammatically as follows. 
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