Question

Tetrahedral complexes are formed:
I: where the central metal has a low oxidation state
II: where ligands are weak field
III: where the electronic configuration of the central metal is, ${d^0},{d^5}$ or ${d^{10}}$
A. I, II, III only correct
B. I, II only correct
C. II, III only correct
D. III only correct

Answer 1

Tetrahedral complexes are formed when the central metal atom is bonded to four ligands, hence having a hybridisation of $s{p^3}$. Thus, these complexes are formed when the metal ion is easy to form. The strength of the ligands in a tetrahedral complex is explained by the crystal field splitting theory.

Complete step by step answer:
First, let us look at some basic properties of tetrahedral complexes to understand the nature of their central metal atoms. Since they have a hybridization state of $s{p^3}$, the d orbitals do not take part in complex formation. This means that the d orbital of the central metal atom must be either empty or fully filled. Hence, the electronic configuration should be either ${d^0}$ (empty) or ${d^{10}}$ (fully filled). Thus, we can straightaway rule out statement III, since tetrahedral complexes cannot be formed with a ${d^5}$ configuration.
Now, since the orbitals involved are $s$ and $p$, the atom/ion should have these orbitals available with the minimum number of electrons. Hence, the parent atom from which these ions are being formed should have a very low number of electrons in its outermost shell, so that the ionisation becomes easy. Hence, since only a few electrons are being removed to create the central metal ion, these complexes always have the central metal ion at a low oxidation state.
A common example is the complex $[Ni{(CO)_4}]$ in which the oxidation state of nickel is $0$
Thus, statement I is correct.
As we now know, there are only a few ligands in a tetrahedral complex (always less than four). According to the crystal field splitting theory, ligands split the d orbital of the central metal ion into two degenerate orbitals, known as ${t_{2g}}$ and ${e_g}$. Due to the fewer number of ligands in tetrahedral complexes, the magnitude of the splitting is less and thus, the energy difference between these levels is also less. Thus, these ligands are termed as weak ligands.
Hence, we have now proved that statement II is also correct.
Therefore, since only statements I and II are correct

So, the correct answer is Option B .

Note: Complex formation and ligand behaviour is explained mainly by two theories, the VSEPR model and the Crystal Field Theory. The former talked about the structure and shape of complexes but was unable to explain their wide range of colours. The crystal field theory explains most features brilliantly, but fails to explain covalent character found in some complexes.