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.