Influence of the stereochemistry of water co-ordination to metal(III) cations on the M–O bond length and electronic structure of the cation

Abstract

The structures of CsM(SO₄)₂·12D₂O (M = Mo or Ru) have been determined at 15 K using neutron-diffraction techniques. The structures, which both conform to the β-alum modification, were refined using 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)Å respectively are in agreement with previous studies. The metal(III)–water bond lengths of second-row transition-metal aquaions are more sensitive to the stereochemistry of water co-ordination than are their first-row counterparts. For both molybdenum and ruthenium the stereochemistry of water co-ordination to M^III is trigonal planar with a small, but significant, difference of the angle between the plane of the water molecule and the MO₆ framework [–19.7(1)° for Mo^III and –22.0(2)° for Ru^III]. These results are shown to be in keeping with predictions based on the occupancy of the t_2g(O_h) orbitals. There are minimal structural differences between CsRu(SO₄)₂·12H₂O and its deuteriated analogue, an observation which supports the use of either set of structural data for the interpretation of physical measurements on alums.