Question

The correct statement(s) regarding the binary transition metal carbonyl compounds is/are:
(Atomic numbers: $({\mathbf{Fe}} = {\mathbf{26}},\;{\mathbf{Ni}} = {\mathbf{28}})$):
A.Total number of valence shell electrons at metal centre in $Fe{(CO)_5}$ or $Ni{(CO)_4}$ is 16
B.These are predominantly low spin in nature
C.Metal-Carbon bond strengthens when the oxidation state of the metal is lowered
D.The carbonyl C-O bond weakens when the oxidation state of metal is increased.

Answer 1

To answer this question, you should recall the concept of coordination in carbonyl compounds. Carbonyl complexes show a special property of synergy bonding, which includes a property called ${{d\pi - p\pi }}$ backbonding.

Complete step by step answer:
Synergic bonding refers to the transfer of electrons from ligands to metal and electrons from filled metal orbitals to antibonding orbitals of ligands. This is a bond between carbonyl groups and the central metal. The term Synergic means self-strengthening bond.
Pi-acid ligands refers to those ligands which have a lone pair of electrons to donate to metal atoms and an empty their anti-bonding molecular orbital to back bond with the d-orbital electrons of metal atom. The features of synergy bonds is that: It strengthens the Metal carbonyl but weakens carbon-oxygen bonds. This phenomenon has been proved by the increase in vibrational frequency of metal carbonyl bond in infra-red to a point that is outside the normal range for IR spectrophotometer. It also leads to shortening of the M-C-O bond length. Now analysing each option systematically:
(A) $[Fe{\left( {CO} \right)_5}]\;\& \;[Ni{\left( {CO} \right)_4}]\;$ complexes have 8-electrons in their valence shell (4th shell).
(B) Carbonyl complexes are predominantly low spin complexes due to strong ligand fields.
(C) As electron density increases on metals which is due to lowering of oxidation state on metals, the extent of synergic bonding increases. Hence Metal−Carbon bond strength increases.
(D) When the positive charge on metals increases it increases the extent of synergic bond decreases and hence, C−O bond becomes stronger.

Hence, the correct options are B and C.

Note:
This synergy bonding raises the importance of EAN.
According to the EAN rule, the central atom tends to surround itself with the sufficient number of peripheral atoms (ligands) that the resulting effective atomic number is numerically equal to the atomic number of the noble-gas element found in the same period in which the metal is situated. This rule is generally applied to most of the metal complexes with carbonyl complexes such as carbon monoxide and organometallic compounds. The formula for EAN can be given by the equation:
${\text{EAN = Atomic number }}\left( {\text{Z}} \right){\text{ - Oxidation number} + 2 \times \text{Coordination number}}$.