Experimental analysis of the antibonding trimetal character of the HOMO in the paramagnetic Fischer-Palm Ni3(.eta.5-C5H5)3(.mu.3-CO)2 cluster: synthesis and stereochemical characterization of the electronically equivalent [(.eta.5-C5Me5)CoNi2(.eta.5-C5H5)2(.mu.3-CO)2]- monoanion and neutral Ni3(.eta.5-C5Me5)3(.mu.3-CO)2 analog and of the [Ni3(.eta.5-C5H5)3(.mu.3-CO)2]- monoanion