Chemistry - Coordination Chemistry Question with Solution | TestHub

The question asks which complexes exhibit $\pi$-character in their metal-ligand bonds. This refers to $\pi$-backbonding, where the metal donates electron density from its filled d-orbitals to empty ${\pi }^{\ast }$ (antibonding) orbitals of the ligand. This typically occurs with ligands like CO, CN⁻, and aromatic systems.

A. $[\mathrm{Mo}(\mathrm{CO}{)}_6]$: CO is a strong $\pi$-acceptor ligand, forming significant $\pi$-backbonds with transition metals.

B. $[\mathrm{Cr}(\mathrm{CO}{)}_3({\mathrm{B}}_3{\mathrm{N}}_3{\mathrm{H}}_6)]$: CO is a $\pi$-acceptor. Borazine (${\mathrm{B}}_3{\mathrm{N}}_3{\mathrm{H}}_6$) is an inorganic analog of benzene and can act as a $\pi$-donor or $\pi$-acceptor, contributing to $\pi$-character.

C. $[\mathrm{Sc}(\mathrm{CO}{)}_6{]}^{3+}$: Scandium in a +3 oxidation state ($d^0$) has no d-electrons to donate for $\pi$-backbonding.

D. $[\mathrm{Ni}(\mathrm{CN}{)}_4{]}^{4-}$: CN⁻ is a strong $\pi$-acceptor ligand, similar to CO, and forms $\pi$-backbonds with transition metals.

Therefore, complexes A, B, and D exhibit $\pi$-character between the metal and carbon (or nitrogen in CN⁻) atoms due to $\pi$-backbonding.

The final answer is $\boxed{A,B,D}$.