Structure, covalence and spin polarisation in tris(acetylacetonato)ruthenium(III) studied by X-ray and polarised neutron diffraction

A polarised neutron diffraction (PND) experiment on [Ru(acac)(3)] at 4.18 K and 5 T along the b and c axes yielded 28 and 17 flipping ratios respectively. Single-crystal X-ray structure determinations at 293, 92 and 10.5 K, together with powder neutron diffraction experiments, provided auxiliary data allowing magnetic structure factors to be deduced from the PND experiment. The structure is disordered at all temperatures. There are ordered stacks of molecules along b, which stacks are arranged in the ac plane in two sites of relative displacement 0.4 Angstrom along b. The molecular geometries at the sites are similar and approximately D-3 in symmetry, but the magnetisation data indicate C-2 symmetry The PND data showed significant spin delocalisation by covalence onto the acetylacetonato rings, dominated by spin-polarisation effects, and of C-2 symmetry. However the two data sets differed, with the field along b indicating a (2)A and that along c a B-2 dominated state. This cannot be explained in the ligand-field model which fits the magnetic susceptibility data, indicating that vibronic effects must be explicitly considered. Good-quality ab-initio local density calculations reproduced the molecular geometry but grossly underestimated the spin correlation between metal and rings.