The synthesis of the novel tridentate nitrogen ligand (2-((3,5-dimethyl-N-pyrazolyl)methyl)-1,10-phenanthrolin-9-yl)methyl ethyl ether (dpmp), C21H22N4O, and of its dichloronickel(II) complex are reported. The structure of the nickel(II) complex and that of the trans-diaquanickel(II) complex of the related tetradentate ligand 2,9-bis-(N-pyrazolylmethyl)-1,10-phenanthroline (bpmp), C20H16N6, have been determined by three-dimensional X-ray diffraction methods. The chloride salt of the bpmp complex, [Ni(bpmp)(OH2)2]Cl2.H2O, crystallizes in space group P1BAR of the triclinic system with two molecules in a cell of dimensions a = 8.016(2) angstrom, b = 9.997(2) angstrom, c = 15.015(3) angstrom, alpha = 101.22(3)degrees, beta = 92.50(3)degrees, and gamma = 11 1.20(3)degrees. The structure has been refined to a final value of the crystallographic R factor of 0.0272 based on 3861 observed independent reflections. The geometry at nickel is six-coordinate, pseudooctahedral, with water ligands trans. The dpmp complex [Ni(dpmp)Cl2].CH3CN also crystallizes in space group P1BAR of the triclinic system with two molecules in a cell of dimensions a = 8.478(2) angstrom, b = 9.644(2) angstrom, c = 15.747(3) angstrom, alpha = 80.52(3)degrees, beta = 84.58(3)degrees, and gamma = 70.46(3)degrees. The structure has been refined to a final value of the crystallographic R factor of 0.0652 based on 1655 observed independent reflections. The complex adopts a five-coordinate geometry, the coordination being provided by the two phen nitrogen atoms [N(1) and N(2)] and one pyrazole nitrogen atom [N(3)] from the ligand and by two chloride ions. The geometry at nickel is trigonal bipyramidal, with one phenanthroline and one pyrazolyl nitrogen atom axial while the other phenanthroline nitrogen atom and the chloride ligands are equatorial. The nickel (II) bpmp complex exhibits an irreversible reduction wave at -0.46 V vs Ag/AgCl; the nickel(II) dpmp complex, however, shows a quasi-reversible wave at E1/2 = -0.61 V, attributable to the Ni(II) <-> Ni(I) couple.