The reactions of two isomers of 3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (differing in the orientation of the methyl groups on the chiral carbon atoms), designated as L-B and L-C, with CH3I in the ratio of 1:4 resulted in the substitution of the N(4) and N(11) protons by CH3 groups, forming the dimethyl derivatives L-BZ and L-CZ, respectively. These ligands, on interaction with nickel(II) acetate tetrahydrate and subsequent addition of lithium perchlorate, produce square-planar yellow [NiLBZ][ClO4](2) and orange [NiLC'Z][ClO4](2). These nickel complexes undergo axial ligand addition reactions with NCS-, Cl-, Br-, and I- as X- to form six-coordinate trans-diisothiocyanato, -dichloro, -dibromo, and -diiodo complexes of formula [NiLX2], where L = L-BZ or L-C'Z, and X = SCN, Cl, Br, or I. All these compounds have been characterized on the basis of analytical, spectroscopic, conductometric, and magnetochemical data. The structures of L-BZ and two variants of [Ni"L-BZ"][ClO4](2) (crystallizing in the space group P2(1)/n and Pn, respectively; "L-BZ" symbolizes partially methylated ligand) have been determined by single-crystal X-ray analyses. The antifungal and antibacterial activities of these compounds have been studied against some phytopathogenic fungi and bacteria.