Two isomers of the square-planar nickel(II) complex of 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradecane (HTIM) were prepared from the nickel(II) complex of 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,-10-tetraene (TIM) by NaBH4 reduction in aqueous solution. The properties of Ni(II) complexes, Ni(I) complexes, and Ni(I)-CO complexes with a series of 14-membered macrocyclic N-4 ligands were studied by means of UV-vis, NMR, and IR spectroscopy. The CO binding constants range from 1.3 x 10(2) to 2.8 x 10(5) M(-1) at 25 degrees C in acetonitrile. The crystal structures of C-RSSR-[Ni(I)HTIM]ClO4 (1), C-RSSR-[Ni(II)HTIM](ClO4)(2) (2), C-RRSS-[Ni(II)HTIM](ClO4)(2) (3) and [Ni(II)TIM](ClO4)(2) (4) have been determined from single-crystal X-ray diffraction data collected by using Mo Ka radiation. Crystallographic data are as follows : (1) trigonal space group R ($) over bar 3 with a 11.971(3) Angstrom, alpha = 107.46(2)degrees, V = 1410.3(5) Angstrom(3), Z = 3; (2) triclinic space group P ($) over bar 1 with a = 8.503(2) Angstrom, b = 8.438(2) Angstrom, c = 8.028(2) Angstrom, alpha = 102.63(1)degrees, beta = 104.81(1)degrees, gamma = 101.76(1)0, V= 522.4(4) Angstrom(3), Z = 1; (3) monoclinic space group P2(1)/c with a = 8.307(4) Angstrom, b = 12.521(6) Angstrom, c = 10.587(6) Angstrom, beta = 103.66(2)degrees, V = 1070(2) Angstrom(3), Z = 2; (4) orthorhombic space group Pnma with a = 13.189(2) Angstrom, b = 11.672(2) Angstrom, c = 13.757(3) Angstrom, V = 2118(1) Angstrom(3), Z = 4. All complexes have square-planar’geometry and the stereochemistry of the four coordinating N atoms in complexes (1), (2), and (3) is Trans III. Although the two sets of Ni-N distances of 2.083(3) and 2.053(3) Angstrom for the Ni(I) complex, (I), are much longer than those of 1.969(1) and 1.948(2) Angstrom for the corresponding Ni(II) complex, (2), distortion of the macrocycle core was not observed upon the reduction of the metal center. The structural changes associated with the reduction of Ni(II) to Ni(I) in square-planar complexes can be classified in three ways : (1) expansion of Ni-N distances; (2) distortion of nickel core; (3) both expansion and distortion.