The dependence of the properties of mixed ligand [Ni^II ₂L(μ-O₂CR)]⁺ complexes (where L^2- represents a 24-membered macrocyclic hexaamine-dithiophenolato ligand) on the basicity of the carboxylato coligands has been examined. For this purpose 19 different [Ni^II₂L(μ-O₂CR)]⁺ complexes (2-20) incorporating carboxylates with pK_b values in the range 9 to 14 have been prepared by the reaction of [Ni^II ₂L(μ-Cl)]⁺ (1) and the respective sodium or triethylammonium carboxylates. The resulting carboxylato complexes, isolated as ClO₄^- or BPh₄^- salts, have been fully characterized by elemental analyses, IR, UV/vis spectroscopy, and X-ray crystallography. The possibility of accessing the [Ni^II ₂L(μ-O₂CR)]⁺ complexes by carboxylate exchange reactions has also been examined. The main findings are as follows: (i) Substitution reactions between 1 and NaO₂CR are not affected by the basicity or the steric hindrance of the carboxylate. (ii) Complexes 2-20 form an isostructural series of bisoctahedral [Ni^II₂L(μ-O ₂CR)]⁺ compounds with a N₃Ni(μ-SR) ₂(μ-O₂CR)NiN₃ core. (iii) They are readily identified by their V_as(CO) and V_s(CO) stretching vibration bands in the ranges 1684-1576 cm^-1 and 1428-1348 cm ^-1, respectively. (iv) The spin-allowed ³A_2g → ³T_2g (V₁) transition of the NiOS ₂N₃ chromophore is steadily red-shifted by about 7.5 nm per pK_b unit with increasing pK_b of the carboxylate ion. (v) The less basic the carboxylate ion, the more stable the complex. The stability difference across the series, estimated from the difference of the individual ligand field stabilization energies (LFSE), amounts to about 4.2 kJ/mol [Δ_LFSE(2,18)]. (vi) The "second-sphere stabilization" of the nickel complexes is not reflected in the electronic absorption spectra, as these forces are aligned perpendicularly to the Ni-O bonds. (vii) Coordination of a basic carboxylate donor to the [Ni ^II₂L]²⁺ fragment weakens its Ni-N and Ni-S bonds. This bond weakening is reflected in small but significant bond length changes. (viii) The [Ni^II₂L(μ-O₂CR)] ⁺ complexes are relatively inert to carboxylate exchange reactions, except for the formato complex [Ni^II₂L(μ-O ₂CH)]⁺ (8), which reacts with both more and less basic carboxylato ligands.