Spin density distribution in mononuclear Rh(0) complexes: A combined experimental and DFT study

The paramagnetic complex [Rh(trop2dach)]radical dot2 was obtained by reduction of the almost planar 16-electron cationic precursor complex, [Rh(trop2dach)]+1 and characterized by EPR spectroscopy [g11 = 2.069, g22 = 2.014, g33 = 1.964, giso = 2.016; A(Rh) = (<40, 29, 30)]. The unobservable small nitrogen hyperfine coupling and DFT calculations show that most of the spin density is localized on the hydrocarbon ligand framework and only about 35% on the metal center. DFT calculations on various 17 electron rhodium complexes with carbonyl, olefine, or phosphane ligands like [Rh(CO)4]radical dot, [Rh(cod)2]radical dot, and [Rh(dppe)2]radical dot reveal that in none of these the spin density at the metal center exceeds 45%. That is all formally Rh(0) complexes reported to date are better described as highly delocalized radicals and an assignment of the formal metal oxidation state is not meaningful.