Mercury-bridged transition-metal clusters. 5. Synthesis and structure of (.mu.3-.eta.2-C2-tert-Bu)(CO)9M3(.mu.3-Hg)M' [M = Ru, Os; M' = Fe(.eta.5-C5H5)(CO)2, Ru(.eta.5-C5H5)(CO)2, Mo(.eta.5-C5H5)(CO)3, Co(CO)4] and [(.mu.-H)(.mu.3-S)(CO)9Os3]2(.mu.4-Hg)