A Tetranuclear Rhodium Complex with a .mu.4-S Ligand, [{Cp*2Rh2(.mu.2-CH2)2}2(.mu.4-S)]2+ (Cp* = .eta.5-C5Me5), Obtained from Stepwise Abstraction of the SH-Ligand from a Dinuclear Rhodium Dihydrosulfide Precursor by Ag+