Given below are two statements: one is labelled as "Assertion A" and the other is labelled as "Reason R"
Assertion A : In the complex Ni(CO)₄ and Fe(CO)₅, the metals have zero oxidation state.
Reason R : Low oxidation states are found when a complex has ligands capable of π-donor character in addition to the σ-bonding.
In the light of the above statements, choose the most appropriate answer from the options given below

Solution

Chemistry ma’am · 4 steps

CO is neutral, so A holds, but CO keeps metals at zero by accepting π electrons, not donating them.

1. Check Assertion A. CO is a neutral ligand and both complexes carry no overall charge. For Ni(CO)₄, x + 4(0) = 0 gives x = 0, and the same logic gives Fe a 0 state in Fe(CO)₅. A is correct.
2. How CO bonds. CO gives its carbon lone pair to the metal, which forms the σ bond. The metal then pushes electron density from its filled d orbitals back into the empty π^* orbitals of CO. This back bonding makes CO a π-acceptor.
3. Check Reason R. A metal in the zero oxidation state is electron rich. A π-acceptor ligand drains that extra density and stabilises it. A π-donor would pile on even more electron density, so π-donors suit high oxidation states instead. R says π-donor, so R is not correct.
4. So. A is correct and R is not correct, which is option (b).

The idea to keep. Low oxidation states need ligands that pull electron density off the metal: π-acceptors like CO, CN⁻ and PR₃. π-donors such as Cl⁻ or O²⁻ go with high oxidation states.

Answer from the NTA final answer key. Solution by PrepWiser.