Dimethyl sulfoxide solvates of the aluminium(III), gallium(III) and indium(III) ions.: A crystallographic, EXAFS and vibrational spectroscopic study

The isostructural hexakis( dimethyl sulfoxide)-aluminium(III), -gallium(III) and -indium(III) iodides crystallise in the trigonal space group R(3) over bar ( no. 148), Z = 3, at 295 +/- 1 K. The metal ions are located in a (3) over bar symmetry site with M-O bond distances of 1.894(4), 1.974(4) and 2.145(3) Angstrom, and M-O-S bond angles of 127.1(3), 124.1(3) and 123.1(2)degrees, for M = Al, Ga and In, respectively. The unit cell parameters are a = 10.762(2), c = 24.599(3) Angstrom, V = 2467.2(5) Angstrom(3) for [Al(OS(CH3)(2))(6)]I-3, a = 10.927(2), c = 23.868(4) Angstrom, V = 2468.1(6) Angstrom(3) for [Ga(OS(CH3)(2))(6)]I-3, and a = 11.358(2), c = 21.512(4) Angstrom, V = 2403.5(7) Angstrom(3) for [In(OS(CH3)(2))(6)]I-3. The increasing compression of the octahedral MO6 coordination entities along one three-fold axis for M = Al, Ga and In, respectively, explains why the largest ion indium(III) has the smallest unit cell volume. EXAFS measurements on the dimethyl sulfoxide solvated gallium(III) and indium(III) ions in solution and in the solid perchlorate and trifluoromethanesulfonate salts, show similar bond distances as in the solid iodide solvates. Raman and infrared spectra have been recorded for the hexakis( dimethyl sulfoxide) metal(III) iodides and the nature of the metal-sulfoxide bond has been evaluated by normal coordinate methods. The symmetric and asymmetric M-O stretching modes correspond to the vibrational frequencies 465 and 540 cm(-1) for [Al(OS(CH3)(2))(6)]I-3, 491 and 495 cm(-1) for [Ga(OS(CH3)(2))(6)]I-3, and 444 and 440 cm(-1) for [In(OS(CH3)(2))(6)]I-3, respectively.