Question

The Racah parameter B is 460 cm-1 in [Co(CN)6]3– and 615 cm-1 in [Co(NH3)6]3+. Consider the nature of bonding with the two ligands and explain the difference in the B value.

Answer 1

When an atom has more than one electron there will be some electrostatic repulsion between those electrons. The amount of repulsion varies from atom to atom, depending upon the number and spin of the electrons and the orbitals they occupy. The total repulsion can be expressed in terms of three parameters A, B and C which are known as the Racah parameters. Racah parameter B, the nephelauxetic effect refers to a decrease in the Racah inter electronic repulsion parameter, that occurs when a transition-metal free ion forms a complex with ligands

When an atom has more than one electron there will be some electrostatic repulsion between those electrons. The amount of repulsion varies from atom to atom, depending upon the number and spin of the electrons and the orbitals they occupy. The total repulsion can be expressed in terms of three parameters A, B and C which are known as the Racah parameters. Racah parameter B, the nephelauxetic effect refers to a decrease in the Racah inter electronic repulsion parameter, that occurs when a transition-metal free ion forms a complex with ligands

These two ligands [Co(CN)6]3– and [Co(NH3)6]3+ are quite different with respect to the types of bonds they form with metal ions. Ammonia and cyanide are both s-bases, but cyanide is also a p-acid. This difference means that NH3 can form molecular orbitals only with the metal eg orbitals, while cyanide can form molecular orbitals with the metal eg and t2g orbitals. The formation of molecular orbitals will have the delocalization of electrons, and this is precisely what the Racah parameter B measures and hence the difference in the values