The structure of the complex [Zn(cyclen)Tu](NO3)(2) (1) is reported (cyclen = 1,4,7,10-tetraazacyclododecane; Tu = thiourea): orthorhombic, space group P2(1)2(1)2(1), a = 11.4170(11) angstrom, b = 12.1995(11) angstrom, c = 12.5299(12) angstrom, Z = 4, R = 0.0504. The coordination of the cyclen is the same as that found for other similar Zn-II complexes, with square pyramidal coordination around the Zn-II and mean Zn-N bond lengths of 2.16 angstrom. The coordinated Tu occupies the axial coordination site, with Zn-S = 2.31 angstrom. The Zn-S-C-N torsion angle, involving the coordinated Tu, of 75.4 degrees is unusually large, because such torsion angles involving coordinated Tu are normally closer to 0 degrees. The bonding between Zn and S is discussed in terms of overlap with the p orbitals on S, which favors the eclipsed (Zn-S-C-N torsion = 00) mode of coordination of Tu. The energies of eclipsed and staggered modes (Zn-S-C-N = 900) of coordination of Tu to metal ions are examined by means of ab initio calculations, using the STO-3G basis set. It is concluded that the rather low formation constant for the Tu complex with Zn-II/cyclen reported in this work was due to steric effects in 1, which prevent the adoption of the lower energy eclipsed conformation. These steric effects, because of clashes that would occur between Tu in the eclipsed conformation and the cyclen ring, cause the coordination of Tu with a higher energy conformation, with Zn-S-C-N = 75.4 degrees. The latter approaches the high energy staggered conformation that has Zn-S-C-N = 90 degrees. log K-1 values for Cl-, Br-, I-, and CN- are reported and shown to be consistent with the binding site on the Zn-II in the Zn-II/cyclen complex being softer in the hard and soft acids and bases (HSAB, Pearson 1997) sense than the Zn-II aqua ion, but not as soft as Zn-II in triaza macrocycles that promote tetrahedral coordination. The change in HSAB character from intermediate in the Zn-II aqua ion to softer in the cyclen complex, and softer still in tridentate N-donor ligands in model complexes, and in the Zn-II active site of carbonic anhydrase as representative of Zn-II metalloenzymes in general, is discussed in terms of the role of such effects in the functioning of metalloenzymes.