Covalency as Source of Intensity for d‐d Transitions in Octahedral Complexes

A pair‐vibrational mechanism is suggested for inducing of spin‐forbidden d‐d transitions in centrosymmetric complexes. Only the vibrational part of this mechanism appeared to be sufficient to account for the observed strong difference in intensity of the corresponding d‐d transitions (spinforbidden included) in different crystals with analogous magnetic properties. This phenomenon is proved to be due to the difference in the degree of covalency of the metal‐ligand bonds and not to the difference in the value of crystal fields in the compounds. Experimental results obtained on the crystals RbMnF₃, CsMnF₃, and RbMnCl₃, (in hexagonal and cubic modifications) are in agreement with the drawn deductions.