Question

Which of the following does not obey 18-electron rule?
A. $Cr{(CO)_6}$
B. $Fe{(CO)_5}$
C. $V{(CO)_6}$
D. $M{n_2}{(CO)_{10}}$

Answer 1

V atom in $V{(CO)_6}$ contains 17 valence electrons. 12 electrons are from 6 CO ligands and 7 electrons are from V. Hence, it does not obey the 18 - electron rule. While $Cr{(CO)_6}$ , $Fe{(CO)_5}$ and $M{n_2}{(CO)_{10}}$ all these complexes contains18 valence electrons on the central metal atom. Thus, they follow 18 electron rules.

Complete step by step answer:
The 18-valence electron (VE) rule states that thermodynamically stable transition metal compounds contain 18 valence electrons consisting of the metal d electrons plus the electrons that are supplied by the metal bound ligands. The counting of the 18 valence electrons in transition metal complexes may be obtained by following either of the two methods of electron counting, the first is - the ionic method and the second method is the neutral method.
Effective Atomic Number = No. of electrons in the central metal atom or ion + No. of electrons donated by ligands
Electron count of central metal atom
In $Cr{(CO)_6}$
$6\left( {Cr} \right) + 6 \times 2\left( {CO} \right) = 18$ electrons
In $Fe{(CO)_5}$
$8\left( {Fe} \right) + 5 \times 2\left( {CO} \right) = 18$ electrons
In $V{(CO)_6}$
$5\left( V \right) + 6 \times 2\left( {CO} \right) = 17$ electrons
In $M{n_2}{(CO)_{10}}$
$7\left( {Mn} \right) + 5 \times 2\left( {CO} \right) + 1 \times 1\left( {Mn-Mn} \right) = 18$ electrons
Thus, $V{(CO)_6}$ does not obey 18-electron rule as V has 17 electrons.

Therefore, the correct answer is option (C).

Note: In general, the conditions favouring adherence to the 18-electron rule are, an electron rich metal (one that is in a low oxidation state) and ligands that are good $\pi$ ‐acceptors
In $M{n_2}{(CO)_{10}}$ molecule, each Mn atom is directly attached to other Mn atom by a Mn – Mn σ-bond which is formed by the linear overlapping of singly occupied ${d_2}$ $s{p^3}$ hybrid orbitals.