Q.Based on VB theory explain why [Cr(NH₃)₆]³⁺ is paramagnetic, while [Ni(CN)₄]²⁻ is diamagnetic.

Valence Bond Theory (VBT), proposed by Linus Pauling, explains the metal-ligand bond as covalent: each ligand donates a lone pair into a vacant HYBRID orbital on the metal, and the number of hybrid orbitals needed equals the coordination number. Hybridisation fixes geometry directly: sp -> linear (CN 2), sp² -> trigonal planar (CN 3), sp³ -> tetrahedral (CN 4), dsp² -> square planar (CN 4), dsp³ -> trigonal bipyramidal (CN 5), d²sp³ or sp³d² -> octahedral (CN 6).

For octahedral complexes specifically, if the (n-1)d orbitals are used in hybridisation the complex is an INNER-orbital / low-spin / spin-paired complex; if the outer nd orbitals are used instead, it is an OUTER-orbital / high-spin / spin-free complex. Strong-field ligands (CO, CN⁻, en, NH₃) force electron pairing and favour inner-orbital complexes; weak-field ligands (like F⁻) do not force pairing and favour outer-orbital complexes. Magnetism follows directly: any unpaired electron makes a complex paramagnetic (spin-only moment μs = √[n(n+2)] BM); fully paired electrons make it diamagnetic. …