Relationships between Structure and Ligand Dynamics in Organometal Clusters

The variable-temperature ¹H and ¹³C NMR spectra of the hydrido organometal clusters HM₃(CO)₉-[CH₃CCC(CH₃)₂] (M = Ru, Os) have been examined. When M ≈ Ru, a dynamic process is observed which averages the magnetically inequivalent methyl groups on the ligand and also results in a twofold symmetry plane at the metal cluster with respect to the carbonyl ligands. These observations are explained in terms of edge hopping of the hydride ligand and a simultaneous “wagging” motion of the organic ligand. For the osmium derivative the barrier to the motion of the organic ligand is higher, but it cannot be established whether it is directly coupled with the edge-hopping of the hydride. For comparison, the variable-temperature ¹³C NMR spectrum of HRu₃(CO)₉(CH₃CCHCCH₃) was investigated. Here, no motion of the organic ligand could be detected although opening of the hydride bridge appears to be occurring as evidenced by the different barriers to axial-radial CO site exchange at the different ruthenium atoms. The phosphine derivative HRu₃(CO)₈P(C₆H₅)₃(CH₃CCHCCH₃), in which the phosphine is bound to the metal atom which is σ-bound to the organic ligand, shows a higher barrier to axial-radial site exchange at the hydride-bridged metal atoms.