A five-coordinate Ni(I) macrocyclic complex, (R,S,R,S)-[Ni(L)(NHC(OH)CH3)]ClO4, where L is 1,3,6,8, 12,15-hexaazatricyclo[13.3.1.1(8,12)]icosane has been prepared by the reduction of Ni(II) complex (R,R,S,S)-[Ni(L)]-(ClO4)(2) . 1/2H(2)O With Na(Hg) in MeCN under a nitrogen atmosphere. In the formation of the complex, the solvent MeCN was hydrated to acetamide and coordinated to the Ni(I) ion. The complex shows rhombic EPR spectra with the powder sample or in the frozen MeCN solution. It equilibrates with four-coordinate (R,R,S,S)-[Ni(L)](+) in MeCN. [Ni(L)(NHC(OH)CH3)]ClO4 (NiC16H35N7O5Cl) crystallizes in the orthorhombic space group Pbca with a = 10.053(5) Angstrom, b = 14.120(3) Angstrom, c = 31.005(3) Angstrom, and Z = 8. The structure was solved by the direct method and refined to R values of R(1) = 0.0500 and omega R(2) = 0.0983 for 2153 unique reflections measured with Mo Ka radiation on a CAD-4 diffractometer. The Ni(I) ion of the complex has square-pyramidal geometry with an acetamide coordinating in the iminol form at the apical position. The complex shows two sets of Ni-N(macrocycle) bond distances : 2.092(3) Angstrom (av) for Ni-N(sec) and 2.135(3) Angstrom (av) for Ni-N(tert), which are significantly longer than those of the square-planar Ni(I) complex (R,R,S,S)-[Ni(L)]ClO4. The Ni-N(iminol) bond distance is 1.969(5) Angstrom. The three bonds of Ni-N-C-O involving an iminol group have multiple bonding character.