Question

The complex $ {\left[ {Co{{\left( {OX} \right)}_3}} \right]^{3 - }} $ has:
(A) $ s{p^3}{d^2} $ hybridisation with four unpaired electrons
(B) $ {d^2}s{p^3} $ hybridisation with no unpaired electron
(C) Square planar geometry
(D) Trigonal planar geometry

Answer 1

Hint :Complex compound consists of a metal atom and ligand. In presence of a strong field ligand, the electrons in metal atoms undergo pairing. Then the complex is known as the inner orbital complex. Every atom tries to attain the nearest noble gas configuration by accepting electrons from ligands.

Complete Step By Step Answer:
Complex compounds are also known as coordination compounds. These are special compounds unlike double salts. As these compounds retain their identity even dissolved in water or any other organic solvents.
Cobalt has an atomic number of $ 27 $ and has $ 27 $ electrons. The ligand is oxalate in the given complex and the charge of oxalate is $ - 2 $ .
The oxidation state of cobalt will be $ x + 3\left( { - 2} \right) = - 3 \Rightarrow x = + 3 $ . Thus, cobalt is in $ + 3 $ oxidation state and loses the electrons from $ 4s $ and $ 3d $ orbitals.
Now cobalt has $ 24 $ electrons and three oxalate ligands donate $ 6 $ electrons. The total electrons involved is $ 36 $ which attains the nearest noble gas configuration.
At first, the $ 6 $ electrons were in $ 3d $ orbital without pairing up of electrons. But when the oxalate ligand is nearer to cobalt, pairing up of electrons takes place. The incoming electrons enter into $ 3d,4s $ and $ 4p $ orbitals lead to the hybridization of $ {d^2}s{p^3} $ without unpaired electrons.
The complex $ {\left[ {Co{{\left( {OX} \right)}_3}} \right]^{3 - }} $ has $ {d^2}s{p^3} $ hybridisation with no unpaired electron.
Option B is the correct one.

Note :
Oxalate is a bidentate ligand as the two oxygen atoms donate the electrons to the central metal atom in the complex compound. It is an intermediate ligand and no pairing up of electrons takes place in complexes. But, in presence of a cobalt atom only it acts as a strong ligand and pairing up of electrons takes place which is an exception and key point.