A series of heterodinuclear bis (mu-hydroxo)chromium(III)nickel(II) complexes was newly prepared: [(phen)(2)Cr(mu-OH)(2)Ni(tpa)](CIO4)(3).0.5H(2)O (1), [(phen)(2)Cr(mu-OH)(2)Ni(Me-tpa)](CIO4)(3)-2H(2)O (2), [(phen)(2)Cr(mu-OH)(2)Ni(Me-2-tpa)](CIO4)(3).2H(2)O (3), and [(phen)(2)Cr(mu-OH)(2)Ni(Me-3-tpa)](CIO4)(3).3H(2)O (4), where phen is 1,10-phenanthroline and tpa, Me-tpa, Me-2-tpa, and Me-3-tpa are tris(2-pyridylmethyl)amine, [(6-methyl-2-pyridyl)methyl]bis(2-pyridylmethyl)amine, bis[(6methyl-2-pyridyl)methyl](2-pyridylmethyl)amine, and tris[(6-methyl-2-pyridyl)methyl]amine, respectively. X-ray crystallography revealed that the structures of 1-4 resemble one another having an edge-shared bioctahedral structure with a Cr(mu-OH)(2)Ni unit (crystal data: 1.C2H5OH, triclinic, P (1) over bar, a = 13.179(4) Angstrom, b = 13.685(4) Angstrom, c = 14.260(4) Angstrom, alpha = 84.95(2)degrees, beta = 77.65(1)degrees, gamma = 90.21(2)degrees, V = 2502(1) Angstrom(3), Z = 2, R = 0.103, R-w 0.097; 2.C2H5OH, triclinic, F (1) over bar, a 13.214(2) Angstrom, b = 13,657(2) Angstrom, c = 14.417(3) Angstrom, alpha = 95.205(5)degrees, beta = 102.583(4)degrees, gamma = 90.720(3)degrees, V = 2527.3(8) Angstrom(3), Z = 2, R = 0.090, R-w = 0.122; 3.C2H5OH, triclinic, F (1) over bar, a = 13.276(2) Angstrom, b = 13,696(2) Angstrom, c = 14.454(2) Angstrom, alpha = 95.640(3)degrees, beta = 102.821(4)degrees, gamma = 90.174(3)degrees, V = 2549.5(6) Angstrom(3), Z = 2, R = 0.087, R-w = 0. 119; 4, triclinic, F (1) over bar, a = 10.8916(g) Angstrom, b = 14.268(2) Angstrom, c = 17.522(2) Angstrom, alpha = 84.498(9)degrees, beta = 74.313(7)degrees, gamma = 72.402(7)degrees, V = 2498.6(5) Angstrom(3) = 2, R = 0.060, R-w = 0.088). Chromium and nickel ions are coordinated by two phen's and Me-n-tpa, respectively, to complete a distorted octahedral coordination sphere. Introduction of the 6-methyl group(s) onto the pyridyl group(s) results in the elongation of the Ni-N bond distances due to an unfavorable steric interaction between the methyl group and the bridging hydroxide group: systematic elongation of the Ni-N bond distances and the Cr...Ni separations accompanied by an increase in the Cr-O-Ni angles was observed as the number of the methyl groups increases. Variable-temperature magnetic susceptibility measurements of 1-4 (4.2-300 K) indicated that magnetic interactions between Cr(III) and Ni(II) ions are systematically modulated from a very weak antiferromagnetic interaction to a ferromagnetic interaction as the number of the methyl groups increases; the exchange integrals J's for 1-4 are estimated to be -1.4, +0.0, +4.1, and +7.4 cm(-1), respectively. The magneto-structural relationship is discussed in terms of the change in the magnetic orbital energies of nickel(II) centers arising from the change in the Ni-N bond distances.