Titanium-activated carbon-carbon bond formation by reductive head-to-head coupling of carbon disulfide: synthesis of [(.eta.5-C5H5)2Ti]2(C2S4) and comparative analysis of this electron-delocalized tetrathiolene-bridged dititanium complex with the electronically equivalent, electron-localized oxalate- and tetra-p-tolyloxalylamidine-bridged dititanium complexes