The isoleptic organonickel complexes [(bpy)Ni(Mes)X] (bpy = 2,2'-bipyridine; Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or l, and for comparison X = OMe and SCN) have been investigated by multiple spectroscopic means. Their structures have been determined in part by single-crystal X-ray diffraction, the full series by extended X-ray absorption fine structure. The long-wavelength charge transfer absorptions (mainly metal-to-ligand charge transfer) obtain contributions from the mesityl coligand but are almost invariable upon variation of X. UV-vis spectroscopy allowed investigation of the solvolysis reaction [(bpy)Ni(Mes)X] + Solv reversible arrow [(bpy)Ni(Mes)(Solv)](+) + X-, which occurs very fast for X = I (k = 0.176(4) M-1 s(-1)) or Br but very slow for X = Cl (k = 5.18(5) x 10(-5) M-1 cm(-1)) or F. Quantum chemical (density functional theory) calculations on the geometry, electronic states, and electronic transitions (time-dependent density functional theory) are very helpful for detailed insight into the role the X coligands play in these complexes. The combination of methods reveals rather strong, highly covalent Ni-X bonds for all halide coligands but marginal pi-donation.