Hydride atom location and atomic dispersion parameter analysis in trimetal clusters using variable temperature X-ray diffraction

Abstract

Variable temperature X-ray diffraction studies were carried out on several μ-imidoyl clusters with the general formula, (μ-H)₂ M ₃(CO)₉-(μ-imidoyl)(X) whereM=Ru or Os,X=Cl, Br, CF₃COO, and imidoyl ligand is

or CH₃C=NCH₂CH₃, in order to analyze ligand atom motion in the solid state and to precisely locate the hydride ligands. An initial study on (μ-H)(μ₃-η²-CH₃C=NCH₂CH₃)Ru₃(CO)₈PPh₃ indicates that the atomic (thermal) dispersion parameters (ADP) of the μ₃-imidoyl ligand atoms can possibly be used to correlate solution ligand fluxionality with ligand-binding forces in the solid-state structures. Difference Fourier analysis techniques were used to locate hydride ligands in several triosmium clusters, and it was observed that if there was a nitrogen atom in the bridging ligand along a metal edge which also contained a hydride bridge then the hydride was asymmetrically bonded (ca. 0.1–0.7 Å) and always closer to the nitrogen bonded metal. In addition, in dihydrido trimetallic clusters, the distance of separation of the two hydride ligands correlates with the tendency of those hydrides to exchange at room temperature. If the angle ϑ, which describes the angle between the planes relating the bridging hydride ligands and the two inclusive metal atoms, is less than about 55 degrees, then the hydrides can undergo site exchange at room temperature on the nmr time scale.