Spin-forbidden transitions in some cis-Cr III A4B2 ions. A failure of spherical parameterization of the interelectron repulsion integrals

Abstract
The 2E4A2(O) luminescence and 4A22E, 2T1 absorption spectra of cis-[Cr(en)2F2]I, cis-[Cr(en)2Cl2]Cl·H2O, cis-[Cr(en)2Br2]Br·H2O and NH4[Cr(en)F4](en = 1,2 diaminoethane, have been measured at temperatures down to 5 K. In each case the two components of the 2E state have been identified and their separation compared with that calculated from the complete weak field matrices of the d3-tetragonal crystal field–inter-electron repulsion–spin-orbit coupling–Trees correction perturbations. For the dichloro- and dibromo- complexes the calculated separation is an order of magnitude smaller than is observed but the discrepancy is less marked for the fluorocomplexes. This is interpreted as evidence for differential metal d-orbital expansion effecting the values of the electron–electron repulsion integrals.

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