Geometrical Difference and Electron Configuration of Lantern-Type Rh₂⁴⁺ and Rh₂⁵⁺ Complexes: X-ray Structural and DFT Study

Oxidation of [Rh2(O2CEt)4(PR3)2] (R: cy = cyclohexyl and Pri = iso-propyl) with ferrocenium ion gave stable salts of their cationic radicals. Their X-ray structures showed ca. 0.048 Å longer Rh−Rh and ca. 0.12 Å shorter Rh−P bonds than those of the diamagnetic neutral complexes. The pyramids of the phosphine ligands of these cationic complexes are more flattened than those of the neutral complexes. These observations and ESR data indicate that the σRhRh singly occupied molecular orbital (SOMO) of these complexes is extensively mixed with the phosphine lone-pair orbitals in the Rh−P σ antibonding phase. X-ray crystallographic structures of [Rh2(mhp)4]+ (mhp = anion of 2-hydroxy-6-methylpyridine) and [Rh2(form)4]+ (form = N,N‘-di-p-tolylformamidinate anion) showed that the removal of a δRhRh* electron results in 0.017−0.043 Å decreases of their Rh−Eq bond distances, where Eq is the ligation atom in the bridging ligand. These results are consistent with the delocalization of the δRhRh* orbital (the SOMO of these cationic radicals) onto the bridging ligands in the π antibonding phase. B3LYP DFT calculations of Rh24+ and Rh25+ model complexes reproduced the ligand dependence of the electron configuration of Rh25+ complexes and geometrical changes accompanying the ionization and showed that the relatively rigid arrangement of the bridging ligands retards the dependence of the Rh−Rh bond length on the electron configuration. Also, the spin population in [Rh2(O2CH)4(PH3)2]+ is extensively delocalized onto the axial ligands, that in [Rh2(HNCHNH)4]+ and [Rh2(HNCHO)4(H2O)2]+ moderately onto the bridging ligands and that in [Rh2(O2CH)4(H2O)2]+ rather localized on the metal atoms.