Examining the out-of-center distortion in the [NbOF5]2- anion

Out-of-center "primary" electronic distortions are inherent to the oxide fluoride anions of the early d0 transition metals. In the NbOF5]2- anion, the Nb5+ moves from the center of the octahedron toward the oxide ligand to form a short Nb=O bond and long trans Nb-F bond. The combined results of single-crystal X-ray diffraction and electronic structure calculations indicate that the primary distortion of the NbOF5]2- anion is affected by the coordination environment that is created by the three-dimensional extended structure. The formation of bonds between an M(L)4(2+) (M = Cd2+, Cu2+; L = 3-aminopyridine, 4-aminopyridine) cation and the oxide and/or trans-fluoride ligands of the NbOF5]2- anion weakens the pi component of the Nb=O bond. At the same time, hydrogen bond interactions between the equatorial fluorides and the aminopyridine groups both lengthen the equatorial Nb-F bonds and can further reduce the symmetry of the NbOF5]2- anion. These combined three-dimensional bond network interactions that serve to lengthen the Nb=O bond and thereby decrease the primary distortion of the NbOF5]2- anion are illustrated in the structures of three new niobium oxide fluoride phases, 4-apyH]2Cu(4-apy)4(NbOF5)2] (4-apy = 4-aminopyridine), Cd(3-apy)4NbOF5 (3-apy = 3-aminopyridine), and Cu(3-apy)4NbOF5, that were synthesized and characterized using X-ray diffraction. Crystal data for 4-apyH]2Cu(4-apy)4(NbOF5)2]: tetragonal, space group /4(1)/ acd (No. 142), with a = 20.8745(8) A, c = 17.2929(9) A, and Z= 8. Cd(3-apy)4NbOF5: tetragonal, space group P4(3) (No. 78), with a = 8.4034(4) A, c = 34.933(3) A, and Z = 4. Cu(3-apy)4NbOF5: monoclinic, space group P2(1)/n (No. 14), with a = 8.822(1) A, b = 16.385(3) A, c = 8.902(1) A, beta = 109.270(3) degrees, and Z = 2.