INFLUENCE OF THE STEREOCHEMISTRY OF WATER COORDINATION TO METAL(III) CATIONS ON THE M-O BOND-LENGTH AND ELECTRONIC-STRUCTURE OF THE CATION

The structures of CsM(SO4)2.12D2O (M = Mo or Ru) have been determined at 15 K using neutron-diffraction diffraction techniques. The structur es, which both conform to the beta-alum modification, were refined usi ng 607 (Mo) and 501 (Ru) unique reflections to give final R factors of 0.013 and 0.016 respectively. The Mo-O and Ru-O bond lengths of 2.094 (1) and 2.010(1) angstrom respectively are in agreement with previous studies. The metal(III)-water bond lengths of second-row transition-me tal aquaions are more sensitive to the stereochemistry of water co-ord ination than are their first-row counterparts. For both molybdenum and ruthenium the stereochemisty of water co-ordination to M(III) is trig onal planar with a small, but significant, difference of the angle bet ween the plane of the water molecule and the MO6 framework [-19.7(1)-d egrees for Mo(III) and -22.0(2)-degrees for Ru(III)]. These results ar e shown to be in keeping with predictions based on the occupancy of th e t2g (O(h)) orbitals. There are minimal structural differences betwee n CsRu (SO4)2.12H2O and its deuteriated analogue, an observation which supports the use of either set of structural data for the interpretat ion of physical measurements on alums.