GEOMETRICAL DIFFERENCE AND ELECTRON CONFIGURATION OF LANTERN-TYPE RH-2(4-2(5+) COMPLEXES - X-RAY STRUCTURAL AND DFT STUDY() AND RH)

Oxidation of [Rh-2(O2CEt)(4)(PR3)(2)] (R: cy = cyclohexyl and Pr-i = i so-propyl) with ferrocenium ion gave stable salts of their cationic ra dicals. Their X-ray structures showed ca. 0.048 Angstrom longer Rh-Rh and ca. 0.12 Angstrom shorter Rh-P bonds than those of the diamagnetic neutral complexes. The pyramids of the phosphine ligands of these cat ionic complexes are more flattened than those of the neutral complexes . These observations and ESR data indicate that the sigma(RhRh) singly occupied molecular orbital (SOMO) of these complexes is extensively m ixed with the phosphine lone-pair orbitals in the Rh-P sigma antibondi ng phase. X-ray crystallographic structures of [Rh-2(mhp)(4)](+) (mhp = anion of 2-hydroxy-6-methylpyridine) and [Rh-2(form)(4)](+) (form = N,N'-di-p-tolylformamidinate anion) showed that the removal of a delta (RhRh) electron results in 0.017-0.043 Angstrom decreases of their Rh -Bh bond distances, where Eq is the ligation atom in the bridging Liga nd. These results are consistent with the delocalization of the delta( RhRh) orbital (the SOMO of these cationic radicals) onto the bridging ligands in the it antibonding phase. B3LYP DFT calculations of Rh-2(4 +) and Rh-2(5+) model complexes reproduced the ligand dependence oi. t he electron configuration of Rh-2(5+) complexes and geometrical change s accompanying the ionization and showed that the relatively rigid arr angement of the bridging ligands retards the dependence of the Rh-Rh b ond length on the electron configuration. Also, the spin population in [Rh-2(O2CH)(4)(PH3)(2)](+) is extensively delocalized onto the axial ligands, that in [Rh-2(HNCHNH)(4)](+) and [Rh-2(HNCHO)(4)(H2O)(2)](+) moderately onto the bridging ligands and that in [Rh-2(O2CH)(4)(H2O)(2 )](+) rather localized on the metal atoms.