I. Demachy et Y. Jean, TRANS OR (UNUSUAL) CIS GEOMETRY IN D(2) OCTAHEDRAL DIOXO COMPLEXES - A DFT STUDY, Inorganic chemistry, 35(17), 1996, pp. 5027-5031
Geometry optimization of the cis and the trans isomers of several octa
hedral dioxo complexes of d(2) electronic configuration are performed
using the gradient-corrected density functional theory (B3LYP and, for
some key structures, BP86). With only monodentate sigma donor ligands
(ReO2(NH3)(4)(+) 7), the usual energy order is found (i.e., the trans
isomer is the most stable). Complexes with a chelating bidentate liga
nd, OsO2(OCH2CH2O)(NH3)(2) (10) and ReO2(HN=CHCH=NH)(NH3)(2)(+) (11),
are used as models for the experimental complexes 5 and 2 in which the
arrangement of the O=M=O unit is trans and cis, respectively. Our cal
culations actually show an inversion of the relative energy of the two
isomers in going from 10 to 11: while the trans isomer is found to be
the most stable in 10, the unusual cis diamagnetic isomer is favored
by about 29 kcal mol(-1) in 11. This result is traced to the geometric
and electronic properties of the bidentate ligand, in particular an a
cute bite angle and good rr acceptor character. In complex 14 with a b
ipyridine chelating ligand (weaker it acceptor than diaza-1,4-butadien
e in 11), this energy difference is, however, reduced to 7.5 kcal mol(
-1) (partial geometry optimization).