Electronic properties of transition-metal complex ions adsorbed on silica gel. Part I. Nickel(II) complexes

Abstract

The electronic spectra of a series of multidentate nitrogen ligand complexes of the nickel(II) ion adsorbed on silica gel in the sodium form by ion exchange have been examined as a function of the degree of hydration of the silica gel surface. The results show that, on adsorption, the nickel(II) species produced are generally similar to those present in the parent aqueous solution. The removal of physically adsorbed water has a marked effect on the electronic spectra of the adsorbed ion; with high ligand: nickel ratios a cis-octahedral chromophore is suggested, but with low ratios a five-co-ordinate chromophore is formed. These processes can be reversed by the readmission of water vapour. The results are interpreted in terms of a competition of the ligands for co-ordination sites on the nickel(II) ion, with the competing ligands being water surface donors and the nitrogen ligands. The surface seems capable of acting as both a bidentate or a terdentate ligand towards the nickel(II) ion. Possible implications to related heterogeneous catalytic systems are considered.