Neutron-diffraction study of a symmetrical ruthenium–hydrogen–ruthenium bond in nonacarbonylhydrido(t-butylethynyl)-triangulo-triruthenium

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Abstract
Neutron-diffraction investigation of the title compound has confirmed the presence of an hydridic hydrogen atom, as suggested by n.m.r. measurements. The compound crystallizes in the triclinic system, space group P with Z= 2 in a unit cell of dimensions a= 9.024(3), b= 12.893(4), c= 9.026(1)Å, α= 105.25(7), β= 100.38(4), and γ= 92.20(8)°. Reflection intensities have been measured (λ 1.212 Å) with a step-scan technique up to 2θ⩽ 68°. The anisotropic least-squares refinement, with 1 339 observed reflections, has led to R 0.028. The hydride atom lies on a pseudo-plane of symmetry out of the cluster plane, and forms a symmetrical bent Ru–H–Ru tricentric bond, confirmed by thermal-motion analysis. Extensions of the two axial Ru–C(carbonyl) bonds trans to the hydride atom clearly intersect at a point very close to the hydride atom : the Ru–H–Ru bond should thus be classified as ‘open’, unlike other W–H–W bonds previously studied. The observation that the Ru3 cluster has equilateral geometry shows that there is an exact balance between the lengthening effect of the hydride atom and the shortening effect of the organic ligand on the bridged Ru–Ru bond.

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