Electronic structure of the MoFe3S4(SH)3−6 ion: A broken-symmetry metal–sulfur cluster

A set of MS-Xα calculations on the MoFe3S4 (SH)3−6 ion is reported. Three levels of theoretical models are considered: spin-restricted, C3v symmetry, which is most easily compared with the traditional ligand-field picture; spin-polarized, C3v symmetry; and spin-polarized, Cs symmetry. An MS=3/2 ground state corresponding closely to the experimental spin-quartet ground state of a single-cube MoFe3S4 cluster is found at the level of the Cs broken-symmetry model. This state has two antiferromagnetically coupled pairs of metal ions: a net majority-spin iron pair and a net minority-spin Mo–Fe pair. The spin densities on the three iron atoms are nearly equal, with 3.06 electrons of spin density on each Fe; Mo carries only a very small net spin, 0.4 electrons. Calculated Mössbauer quadrupole splittings and isomer shifts agree well with experimental results. The model suggests that a quintet ground state should be produced on reduction, consistent with experiment. The present MoFe3S4 cluster calculations are compared extensively with corresponding results for the Fe4S4 cluster.