Question

# Among the following metal carbonyls, the $$C-O$$ bond order is lowest in

A
[Mn(CO)6]+
B
[Fe(CO)5]
C
[Cr(CO)6]
D
[V(CO)6]

Solution

## The correct option is D $$[V(CO)_6]^-$$The overlap of a lone pair on the C atom with the empty hybrid metal orbital forms a metal-to-carbon $$\sigma$$-bond. The transition metal atom in a metal carbonyl fills the non-bonding d-orbitals which are proper symmetry to overlap with the anti-bonding orbitals of CO. The electronic charge is transferred from the filled non-bonding orbitals of the metals to $$\pi^{\star}$$-orbitals of the ligand CO. This reduces the bond order of CO. The $$\pi$$ back-bonding strengthens the M-C bond order and weakens the C-O bond order.CO is a strong ligand, it causes pairing of electrons.$$Mn^+:(z=25)$$$$[Ar]3d^54s^1\Rightarrow [Ar]3d^64s^0$$Three lone pairs are available for back-bonding with vacant orbital of C in CO.$$Fe^0:(z=26)$$$$[Ar]3d^64s^2\Rightarrow [Ar]3d^8$$Four lone pairs are available for back-bonding with vacant orbotal of C in CO.$$Cr^0:(z=24)$$$$[Ar]3d^54s^1\Rightarrow [Ar]3d^64s^0$$Three loan pairs are available for back-bonding with vacant orbital of C in CO.$$V^-:(z=23)$$$$[Ar]3d^44s^2\Rightarrow [Ar]3d^64s^0$$Three lone pairs are available for back-bonding with vacant orbital of C in CO.Minimum back-bonding is possible in $$[V(CO)_6]^{-1}$$, so the M-C bond order is the lowest.Chemistry

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