The Alkyne-Cluster Interaction: Structural, Theoretical, and Spectroscopic Study on the Parallel μ_3-η² Bonding Mode in Trinuclear Carbonyl Clusters of Ruthenium and Osmium

The parallel μ3-η²alkyne-cluster interaction in compounds of formula H₂M₃(CO)₉(RC₂R’) (M - Ru, Os) has been studied by means of X-ray diffraction, ¹H and ¹³C VT (variable temperature) NMR and UV-PES spectroscopy, and CNDO theoretical techniques. Crystals of H₂Os₃(CO)₉(CH₃C₂CH₃) belong to the monoclinic space group P2₁/n with unit cell dimensions of a = 14.538 (4) A, b = 13,004 (4) A, c = 9.817 (3) A, β = 100.17 (5)°, and Z = 4, Full-matrix least-squares refinement converged to R = 4.95% for 3223 unique reflections. Relevant interatomic distances (A): Os(1)-Os(2) = 3.016 (1), Os(1)-Os(3) = 2.763 (1), Os(2)-Os(3) = 2.858 (1), C(42)-C(43) = 1.434 (36), Os(1)-C(42) = 2.067 (24), Os(2)-C(43) = 2.198 (26), Os(3)-C(42) = 2,364 (28), Os(3)-C(43) = 2.263 (27). The small differences observed between the Os(1)-C(42) and Os(2)-C(43) distances are thought to reflect the charge imbalance on the metal atoms. The lowest energy dynamic process observed in the ¹³C NMR spectra is consistent with an oscillatory motion of the alkyne and of the hydride bridging the Os(1)-Os(2) edge (H_a) coupled with the edge hopping of the second hydride (H_b). Subtle electronic changes induced by different substituents on the alkyne cause remarkable differences in the ²J_C,H coupling pattern between H_a and CO ligands. The results of CNDO calculations have been used for the interpretation of the UV-PE spectra and for discussion of the bonding scheme of the studied molecules.