Structural redetermination of Rh4(CO)12 at 293 and 173 K and analysis of the thermal motion in relation to the dynamical behavior

The structure of Rh-4(CO)(12) (1) has been redetermined at room temperature and at 173 K. Crystallographic data for 1 at 293 K are as follows: C12O12Rh4, space group P2(1)/c, a = 9.209(3), b = 11.790(7), c = 17.721(8) Angstrom, beta = 90.46(3)degrees, V = 1924.0(16) Angstrom(3), Z = 4, 5570 reflections to 0 = 30.0 degrees, and R = 0.030 degrees. For 1 at 173 K the data are as follows: space group P2(1)/c, a = 9.127(3), b = 11.672(6), c = 17.492(13) Angstrom, beta = 90.64(5)degrees, V = 1863.3(18) Angstrom(3), Z = 4, 3782 reflections to theta = 26.3 degrees, and R = 0.033. There was no detectable phase change on cooling to 173 K. Crystals of I are twinned by pseudo-merohedry, but a satisfactory refinement was obtained by assuming a (100) twinning mirror plane. TLS analysis of the anisotropic displacement parameters at both temperatures has been undertaken. Investigation of the mean square displacement difference between the observed and the rigid body displacement parameters reveals there is a significant internal motion of the carbonyl ligands relative to the metal skeleton. This motion is consistent with a normal mode or modes of C-3 symmetry.