Question

The most stable complex in the following:
(A) ${{[Pd{{(CN)}_{4}}]}^{4-}}$
(B) $[Fe{{(CO)}_{5}}]$
(C) ${{[Ni{{(CN)}_{4}}]}^{4-}}$
(D) ${{[Ni{{(CN)}_{4}}]}^{3-}}$

Answer 1

Complex is defined as the substance which is an electrically neutral molecule or ion which is formed by the union of simpler substances. The ions are held together by the chemical forces rather than by physical forces.

Complete answer: In the coordination complexes the coordination number is defined as the total number of sigma bonds which are attached to the ligand and the central atom. When the central atom is attached to one and only type of ligand it is called homoleptic complex. When the central atom is attached to more than one type of ligand it is known as heteroleptic complex.
The stability of the complex depends on the strength of the ligand and on the charge of the central metal ion. Both of the properties are directly proportional to each other. In ${{[Pd{{(CN)}_{4}}]}^{4-}}$ the charge on the central ion that is Pd will be
let the charge on Pd be x
$x+4(-1)=-4$
$\Rightarrow x=0$
So the charge on Pd is 0.
In ${{[Ni{{(CN)}_{4}}]}^{4-}}$the charge on Ni will also be 0. But in ${{[Ni{{(CN)}_{4}}]}^{3-}}$the charge on Ni will be +1. In $[Fe{{(CO)}_{5}}]$, CO being a neutral atom has zero charge so the charge on Fe will be zero. CO is the stronger field ligand than CN so CO will have more strength. Hence $[Fe{{(CO)}_{5}}]$ will be the most stable with zero charge on the central ion and with a stronger ligand.

So the correct answer is option(B).

Note: The stability factor also depends on the chelation effect. If the complex contains a chelating ligand then it will be the most stable ligand. Chelating ligand is a ligand which has multiple donor sites, which is able to form five to six membered stable rings with metal.