Trans or (unusual) cis geometry in d(2) octahedral dioxo complexes. A DFT study

Geometry optimization of the cis and the trans isomers of several octahedral 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 ligand, 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 calculations 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 acute bite angle and good rr acceptor character. In complex 14 with a bipyridine chelating ligand (weaker it acceptor than diaza-1,4-butadiene in 11), this energy difference is, however, reduced to 7.5 kcal mol(-1) (partial geometry optimization).