Using valence bond theory of complexes, explain the geometry and diamagnetic nature of the ion `[Cr(NH_3)_6]^(3+)`. Given the atomic number of Cr = 241.

Explanation

Outer electronic configuration of chromium (Z = 24) in ground state `3d^64s^(1)`. Chromium in this complex is in the +3 oxidation state. Chromium achieves the +3 oxidation state by the loss of one 4s electron and two 3d-electrons. The resulting `Cr^(3+)` ion has outer electronic configuration of `3d^8`. The two vacant orbitals, one 4s-orbital and three 4p-orbitals hybridise to give six equivalent c liybridised orbitals. Six pairs of electrons, one from each `NH_3` molecule occupy the six vacant hybrid orbitals so produced. The resulting complex ion has an octahedral geometry. Since the complex contains three unpaired electron, it is paramagnetic. The hybridisation and formation of the complex is illustrated below.