Synthesis and structural characterization of the compounds [WCoAu(.mu.3-CC6H4Me-4)(CO)2(PPh3)(.eta.5-C5Me5)(.eta.6-C2B10H10Me2)] and [MoCo(.mu.-CC6H4Me-4)(CO)2(.eta.4-C4Me4)(.eta.6-C2B10H10Me2)] and studies on related mixed-metal complexes