[Co(2,9-dimethyl-1,10-phenanthroline)(solvent)(4)](2+) ([Co(neo)](2+)) undergoes a significant decrease in symmetry to form an inner-sphere surface complex when grafted directly on preformed silica or introduced during the sol-gel process. The visible and X-ray absorption spectra of the surface adducts are interpreted in terms of a binding mode in which the Co(II) center has a highly distorted pseudo-C-2v symmetry. The interaction of [Co(neo)](2+) with the silica surface was analyzed using an acid-base equilibrium relationship. Half-maximal surface binding was observed at pH ca. 6. Linear fits to the pH dependence data are consistent with inner-sphere binding of a single silanol group to the cobalt center. The formation of the surface species in tetramethoxyorthosilicate (TMOS) sol-gels required approximately 2 equiv of hydroxide anion per cobalt center, suggesting a two-proton-dependent binding event to form a species such as [Co(neo)(SiO)(2)]. Both sol-gel and silica samples showed essentially identical visible and X-ray absorption spectra, indicating formation of very similar surface adducts when the different synthesis procedures were employed. The maximal binding of [Co(neo)](2+) on three silica samples with different pore diameters and surface areas was compared. Increased binding was found to be inversely proportional to surface area and proportional to pore diameter, indicating a preference for less sterically demanding surface sites.