Structural characterization of carbonyl(diphenylacetylene)tris(.eta.5-cyclopentadienylrhodium), Rh3(.eta.5-C5H5)3(CO)(C6H5C2C6H5), and its bis(pentafluorophenyl)acetylene homolog, Rh3(.eta.5-C5H5)3(CO)(C6F5C2C6F5). An assessment of their molecular dissimilarity