Dinuclear iron-molybdenum-sulfur complexes containing the FeS2Mo core. Syntheses, ground-state electronic structures and crystal and molecular structures of the [(C6H5)4P]2[(C6H5S)2FeS2MoS2], [(C2H5)4N]2[(C6H5S)2FeS2WS2], and [(C6H5)4P]2[(S5)FeS2MS2] (M = Mo, W) complexes