Macrocyclic complexes derived from the template synthesis of the cis-dithiolate nickel(II) complex [1,5-bis(mercaptoethyl)-1 ,5-diazacyclooctanato]nickel(II), I, and [N,N'-bis(2,2-dimethyl-2-mercaptoethyl)-1,5-diazacyclooctanato]nickel(II), I*, with S to S linkers of C-2, II*Br-2, C-3, III*Br-2, and C-4, IV*Br-2, were synthesized and fully characterized as square planar complex ions. The systematic enlargement of the N2S2 cavity size correlates with electrochemical measurements which find the Ni-II/I couple to be more accessible through the series, while the Ni-III oxidation state is stabilized in the smaller cavities, maintaining an approximate 2 V difference between the Ni-III/II and Ni-II/I couples. The dithioether compound [I*Me-2]I-2 shows the most positive NiII/I potential (-412 mV vs NHE, in CH3CN solution), consistent with the greater flexibility of the open-chain N2S2 ligand. While the reduced macrocycles show little tendency to react with carbon monoxide, EPR, electrochemical, and infrared spectroscopic studies of the Ni-1 state of the open chain ligand complex support the reversible binding of CO in GO-saturated solutions. Cyclic voltammetry shows EC' activity in the production of H-2 from HBF4 solutions in the presence of the macrocycle complexes and is interpreted in terms of an inner-sphere Ni-III-H intermediate. Crystal data for II*Br-2, III*Br-2, and IV*Br-2 are as follows : II*Br-2 . 3/2H(2)O crystallizes in the monoclinic system, space group P2(1)/c with cell constants a = 7.900(5)Angstrom, b = 18.41(2) Angstrom, c = 15.612(14) Angstrom, beta = 97.53(6)degrees, V = 2251(3) Angstrom(3), and Z = 4, R = 0.0972, R-w = 0.186; for III*Br-2 . 3/2 H2O, monoclinic system, space group P2(1)/c with cell constants a = 18.821(5) Angstrom, b = 7.844(2) Angstrom, c = 32.968(8) Angstrom, beta = 97.84(2)degrees, V = 4824(2) Angstrom(3), and Z = 8 and R = 0.077, R-w = 0.125; for IV*Br-2 . H2O, orthorhombic system, sparse group Pnma with cell constants a = 18.562(3) Angstrom, b = 15.140(3) Angstrom, c = 9.703(3) Angstrom, V = 2726.8(11) Angstrom(3), and Z = 4 and R = 0.044, R-w = 0.099.