Coordinative saturation of cationic niobium– and rhodium–argon complexes

Mass spectra of Nb+ and Rh+ complexes with argon ligands exhibit `magicʹ 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 d4 ions and octahedral for the d8 species, support this interpretation and show that rare gas complexes of d4 metal ions fit perfectly well into the coordination chemical framework based on the Jahn–Teller effect.