Crystalline KTiOPO4 (KTP:,, an inorganic nonlinear optical material with a waveguide figure-of-merit that is twice that of other mixed-metal oxides, contains helical chains of TiO4/2O2/2 octahedra in which a long, short Ti-O bond motif results in a net c-directed polarization. The alternating long and short Ti-O bonds that occur along these chains are the major contributors to the large nonlinear optic and electrooptic coefficients. Analogous chains have been constructed using dichromate [Cr2O7](2-)anions and [M(py)(4)](2+) (M = Cu, Zn) cations; these new transition metal oxides crystallize in the same space group as KTP. Crystal data for Cu(py)(4)Cr2O7: orthorhombic, space group Pna2(1) (No. 33), with a = 15.941(7) Angstrom, b = 16.324(3) Angstrom, c =8.557(2) Angstrom, and Z = 4; for Zn(py)(4)Cr2O7, orthorhombic, space group Pna2(1) (No. 33), with a = 16.503(1) Angstrom, b = 16.005(1) Angstrom, c = 8.8130(5) Angstrom, and Z = 4; for Cd(py)(4)Cr2O7, monoclinic; space group C2/c (No. 15), with a = 14.8034(9) Angstrom, b = 11.1847(7) Angstrom, c = 15.788(1) Angstrom. beta = 110.023(1)degrees, and Z = 4.