Synthesis and structural-bonding analysis of the 50/49/48-electron triangular metal [Co3(.eta.5-C5H4Me)3(.mu.3-S)2]n series in (n = 0,1+,2+): singlet-triplet solution equilibrium of the solid-state diamagnetic 50-electron Co3(.eta.5-C5H4Me)3(.mu.3-S)2 cluster