Influence of the stereochemistry of water co-ordination 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 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 MIII 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 MoIII and –22.0(2)° for RuIII]. These results are shown to be in keeping with predictions based on the occupancy of the t2g(Oh) 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.