X-Ray and neutron diffraction studies on [Ru4(CO)8(µ-H)4{P(OCH3)3}4] at 293 and 20 K : characterisation of the vibrational behaviour of two-co-ordinate hydrogen atoms

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Abstract
X-Ray and neutron diffraction studies on the complex [Ru4(CO)8H4{P(OCH3)3}4] have been carried out at 293 and 20 K respectively. At room temperature the structure is monoclinic, space group P21/c with cell parameters a= 15.700(4), b= 11.775(3), c= 21.331(5)Å, β= 100.49(4)°, and Z= 4. At 20 K the structure is triclinic, space group P with a= 15.371(3), b= 11.508(2), c= 21.079(3)Å, α= 90.07(1), β= 100.09(1), γ= 92.09(2)°, and Z= 4. The X-ray determination shows the molecular structure to consist of distorted tetrahedral Ru4 units substituted so that each ruthenium atom is bound to two terminal carbonyl ligands, one trimethyl phosphite ligand, and two hydride ligands. Despite positional disorder affecting four methoxy groups, the µ-hydride ligands spanning four edges of the Ru4 tetrahedron were located and refined. At low temperature the structure is ordered with two molecules per asymmetric unit, and the neutron analysis provides precise characterization of the molecular structure. All the Ru–H–Ru bridges are symmetrical with mean Ru–H = 1.773(2)Å and show closed three-centre two-electron bonding. Final agreement indices are R(F)= 0.043, R′(F)= 0.038 for 6 647 unique X-ray data, and R(F)= 0.048, R′(F)= 0.038 for 8 601 unique neutron data. Analysis of the vibrational behaviour of the hydride hydrogen atoms as determined in the neutron analysis indicates that this motion is consistent with these hydrogens residing in a single-minimum potential well provided by symmetric three-centre, two-electron Ru–H–Ru interactions.

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