Transition-metal complexes with sulfur ligands. 57. Stabilization of high-valent iron(IV) centers and vacant coordination sites by sulfur .pi.-donation: syntheses, x-ray structures, and properties of Fe("S2")2(PMe3)n (n = 1, 2) and (NMe4)Fe("S2")2(PMe3)2.cntdot.CH3OH ("S2"2- = 1,2-benzenedithiolate(2-))