The syntheses, X-ray structures, and kinetic measurements of pyRh(DH)₂R complexes, where py = pyridine, DH = monoanion of dimethylglyoxime, and R = Me (I), Et (II), and Prⁱ (III), were described.
The geometry of the Rh(DH)₂ moiety, almost identical in all three complexes, is characterized by mean Rh-N equatorial distances ranging from 197.3(7) pm (I) to 198.1(5) pm (II) and mean O...O distances in the range from 265(1)pm (I) to 268(1) pm (III).
Comparison with alkylcobaloximes indicates an increase of 20 pm for the latter and 10 pm for the former. The axial Rh-C and Rh-N distances lengthen with the increasing bulk and the sigma-donating ability of the alkyl group, respectively. The Rh-C distances range from 206.3(5)pm (I) to 210.7(5) pm (III) and the Rh-N (axial) distances from 222.0(3) pm (I) to 223.0(4) pm (III).
The trend of rate constants for the pyridine displacement is that expected on the basis of the increasing sigma-donor power of the alkyl group.
Comparison with the analogous cobaloximes indicates that the structural parameters, mainly determined by the electronic factors of the alkyl group, follow a similar trend both in rhodoximes and cobaloximes. Furthermore, the smoother variation of the Rh-R bond lengths with the increasing bulk of R with respect to that in alkylcobaloximes suggest that the coordination of ligands around the Rh center is less hindered than that around Co.