Low-temperature (20 K) neutron diffraction study of [K(crypt-222)]+[Cr2(CO)10(.mu.-H)]-. Influence of the lattice environment and the K+ ion on the anion's metal carbonyl framework and the ordered chromium-hydrogen-chromium bond

Abstract

A combination room-temperature x-ray and low-temperature (20 +- 1 K) neutron diffraction study of (K-(C/sub 18/H/sub 36/N/sub 2/O/sub 6/))/sup +/(Cr/sub 2/(CO)/sub 10/(..mu..-H))/sup -/ has been performed to determine the anion's configuration in the absence of crystallographic site symmetry constraints. Although the K/sup +/ ion is surrounded by the cryptate molecule, the ion influences the anion's solid-state structure. The relatively close separation of 2.966 (3) A between the K/sup +/ ion and 01 of the anion is nearly equal to the sum of the corresponding ionic radii of ca. 2.9 A and thereby suggests the presence of an appreciable cation-anion interaction. Although this attractive interaction does not noticeably perturb the local C/sub 4 sigma/ symmetry of the respective Cr(CO/sub 5/ fragment, it causes the anion's eclipsed metal carbonyl structure to undergo a twisting distortion toward a previously unobserved bent structure. The two independent Cr(CO)/sub 5/ groups of the anion are rotated ca. 19/sup 0/ with respect to each other about the Cr-Cr line. This configurational change in the metal carbonyl framework is accompanied by a reduction of the Cr-Cr separation to 3.300 (4) A and the Cr-H-Cr bond angle to 145.2 (3)/sup 0/, which collectively imply an increase in the metal-metalmore » bonding component of the closed three-center, two-electron Cr-H-Cr bond. The ordered bridging hydrogen atom resides in a symmetrical electronic environment as suggested by the two independent, but equivalent, Cr-H separations of 1.735 (5) and 1.723 (5) A. These Cr-H distances are comparable in magnitude to the corresponding distances in the (Et/sub 4/N)/sup +/ and ((Ph/sub 3/P)/sub 2/N)/sup +/ salts. The neutron diffraction data were measured with an automated four-circle diffractometer at the Brookhaven high-flux beam reactor.« less