SCF–Xα scattered wave MO studies of the electronic structure of ferrous iron in octahedral coordination with sulfur

SCF–Xα scattered wave calculations are presented for the FeS10−6 cluster at Fe–S distances of 2.50, 2.38, and 2.26 Å for quintet and singlet spin states and the transition state intermediate between them. These results, together with those of recent calculations on S−12, are used to interpret the x-ray and optical spectra of FeS2 (pyrite) and FeS (troilite). A crossover of the internal energies of the quintet and singlet states of FeS10−6 is predicted at R (Fe–S) =2.28 Å and the ease of spin pairing in FeS is related to its high compressibility and its large volume change upon spin pairing.