A vibrational study of the diosmacyclobutene complex Os2(CO)8(μ2-η1,η1-C2H2):: The use of organometallic complexes as vibrational models for chemisorbed ethyne

The vibrational frequencies of the diosmacyclobutene ring in Os-2(CO)(8)(mu(2)-eta(1), eta(1)-C2H2) (1) and its dideuterated isotopologue Os-2(CO)(8)(mu(2)-eta(1), eta(1)-C2D2) (1-d(2)) have been measured and assigned. A normal coordinate analysis shows that the C=C bond is weakened relative to those in ethene and cis-disubstituted ethenes, whereas the Os-C bonds are stronger than M-C sigma-bonds in related complexes, reflecting back-donation from the osmium d-orbitals into the carbon pi*-orbitals in 1. The vibrational frequencies of 1 and other ethyne complexes are compared with the vibrational frequencies of ethyne chemisorbed on metal and silicon single-crystal surfaces.