Coordinative saturation of cationic niobium- and rhodium-argon complexes

Mass spectra of Nb+ and Rh+ complexes with argon ligands exhibit 'magic' peaks Nb+Ar4 and Rh+Ar6, similar to observations for V+Ar4 and Co+Ar6, indicating coordinative saturation. A consistent explanation is obtained by assuming that the rare gas ligands seek out electron density minima in the valence shell of the ion, which permit a closer approach to the metal core and a stronger charge-induced dipole bond. Ab initio density functional calculations, which predict stable square planar complexes for the d(4) ions and octahedral for the d(8) species, support this interpretation and show that rare gas complexes of d(4) metal ions fit perfectly well into the coordination chemical framework based on the Jahn-Teller effect. (C) 1997 Elsevier Science B.V.