Question

The π acid ligands donate their lone pairs to the metal to form a normal σ bond with the latter in addition to it, the vacant orbitals accept e− from the filled metal orbitals to form a type of π−bond which supplements the σ bond.

Which of the following has lowest C-O bond length ?

• A. $\left[Ni\right(CO{)}_4]$
• B. $\left[Co\right(CO{)}_4{]}^{-}$
• C. $\left[Fe\right(CO{)}_4{]}^{2-}$
• D. $\left[Mn\right(CO{)}_6{]}^{+}$

Answer 1

The correct option is D $\left[Mn\right(CO{)}_6{]}^{+}$

This type of bonding where a metal is bonded with the carbon of a carbonyl compound is known as synergic bonding. In a metal carbonyl, the M-C bond has both σ and π character. The ligand donates eΘ pair to the central metal atom, and simultaneously accepts eΘ pair from central metal atom through synergic bonding. Here, all the options are examples of organometallic compounds. Due to synergic bonding, the metal carbon bond strength increases and bond length decreases due to the partial double bond character. Thus, the C-O bond strength decreases and bond length increases and bond order of CO decreases as the number of antibonding electrons increase. Out of the four options, the option D has Mn+, implying that the electron donating nature among the options is minimum. Therefore, tendency of synergic bonding is minimum, and thus M-C bond length is maximum implying that C-O bond strength is maximum and C-O bond length is minimum.