SPIN STATE STABILITIES AND DISTORTIONS OF THE NOVEL MN3(6-) (M = V, CR, FE) IONS

The electronic structures of the MN36- (M = V, Cr, Fe) ions are examined with the intent of understanding their structures and spin states. The CrN36- ion found in Ca3CrN3 has a unique low-spin electronic state, as judged by the magnetic properties reported by DiSalvo and co-workers. The ion's reported planar C2v geometry is entirely consistent with a Jahn-Teller distortion that is expected on the basis of simple theory. Having adopted a Li6MN3 model to study these ions by use of ab initio methods, for both the Cr and Fe systems we find thc D3h quartet state that is most stable at the SCF level gives way to a C2v doublet ground state with the inclusion of a modest degree of correlation (SD-CI). Thc diamagnetism and structure of the recently synthesized VN36- ion is well accounted for in our treatment. Bond lengths for the optimized geometry of Li6CrN3 are found to be in good agreement with the reported C2v structure in Ca3CrN3. In contrast, the D3h geometry reported for the FeN36- ion [in (Ca3N)2[FeN3] and Ba3FeN3] is at odds with an expected Jahn-Teller instability, if the FeN36- ion has a low-spin doublet ground state as we predict. It is likely that the observed D3h structure of FeN36- ions are the result of dynamic Jahn-Teller effects or stabilization by interionic interactions.