Ruthenium-platinum carbonyl cluster complexes with bridging methylene, methylidyne, and carbido ligands; crystal structures of the compounds [Ru2Pt(μ-CH2)(μ-CO)(CO)2{P(cyclo-C6H11)3}(η-C5H5)2], [Ru2Pt2(μ-H)(μ4-CH)(μ-CO)(CO)2(PPr3i)2(η-C5H5)2], and [Ru2Pt2(μ-H)2(μ4-C)(μ-CO)2(PPr3i)2(η-C5H5)2]