Abstract
The crystal structures of the title complexes have been determined by single-crystal X-ray diffraction techniques. The cobalt complex crystallizes in space group Bl, with a = 8.151 (4) Á, b = 14.067 (7) Á, c = 14.163 (7) Á, a = 106.78 (10)°, ß = 93.45 (10)°, and = 89.56 (10)°; block-diagonal least-squares refinement led to a conventional R value of 0.046 using 2068 reflections (/ > 3 (/)). The chromium complex is monoclinic, space group P2t/c, a = 11.537 (5) Á, b = 8.134 (3) Á, c = 15.191 (6) Á, and ß = 92.93 (7)°, and full-matrix least-squares refinement of all 2365 reflections led to R = 0.061. Despite rather different packing arrangements, the configurations of the cobalt and chromium molecular cations are remarkably similar, lending credence to the hypothesis that observed distortions arise from intramolecular contacts. Both cations are severely distorted, with M-N-C and N-M-L angles deviating up to 15 and 6.6°, from “expected” tetrahedral and octahedral values, respectively. Further, inspection of intramolecular nonbonded contacts shows the cobalt complex to be considerably more strained. The structural data are in accord with the behavior of the complexes in simple octahedral substitution reactions. Thus, when methylamine is substituted for ammonia in MCNE^X24· (M = Co, Cr), kinetic steric effects are observed. The magnitude of the observed structural steric effects correlates well with (a) the magnitude of the observed kinetic steric effects and (b) the established mechanism for acid or base hydrolysis of chromium(III) and
cobalt(III) amine and ammine complexes.