Far infra-red spectra and lattice vibrations of hexanitrocobalt (III) complex salts and ammine complex salts

Far infra-red spectra of the hexanitrocobalt (III) complex salts, M₃[Co(No₂)₆] (M: Na, K, Rb and Cs), and the hexammine complex salts, [Co(NH₃)₆]X₃ (X: Cl, Br and I) and [Ni(NH₃)₆]Cl₂, have been measured down to 25 cm⁻¹. In addition to the intramolecular vibrations in the complex ion (the metal—ligand stretching and deformation vibrations associated with the co-ordination bond), the lattice vibrations due to the interaction between the complex ion and the outer ion have been observed in the region below 150 cm⁻¹.

A normal co-ordinate analysis of the crystal as a whole has been performed and the interaction potential constants as well as the intramolecular potential constants have been obtained by using the observed frequencies for both of the lattice vibrations and the intramolecular vibrations. It has been found that the lattice frequencies may be explained by taking into consideration the interaction between the atoms or ions whose interatomic distances are shorter than 3·5 Å. It has been shown that some of the intramolecular potential constants should be somewhat modified by the introduction of the lattice vibrations, but the metal—ligand stretching force constant may be almost accurately obtained without considering the lattice modes.

Far infra-red spectra of the halogenoammine cobalt (III) complex salts have been also measured and several bands attributed to the lattice vibrations have been obtained.