Synthesis, stereochemical analysis, and ligand-transformation reactions of new classes of bicapped tricobalt clusters, [Co3(.eta.5-C5H5-xMex)3(.mu.3-X)(.mu.3-Y)]n (x = 0, 1, 5), containing mixed .pi.-acceptor X and .pi.-donor Y capping ligands (X = CO, NO; Y = NSiMe3, NC(O)NH2, NH): a rational preparative route to nitrene (.mu.3-NH) clusters and the unprecedented direct exchange of an isoelectronic .mu.-NO+ ligand for a .mu.-CO ligand