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 d(0) 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](2)[Cu(4-aPY)(4)(NbOF5)(2)] (4-apy = 4-aminopyridine), Cd(3-apy)(4)NbOF5 (3-apy = 3-aminopyridine), and Cu(3-apy)(4)NbOF5, that were synthesized and characterized using X-ray diffraction. Crystal data for [4-apyH](2)[Cu(4-aPY)(4)(NbOF5)(2)]: tetragonal, space group /4(1)/ acd (No. 142), with a 20.8745(8) Angstrom, c 17.2929(9) Angstrom, and Z = 8. Cd(3-aPY)(4)(NbOF5)(2)]: tetragonal, space group P4(3) (No. 78), with a 8.4034(4) Angstrom, c 34.933(3) Angstrom, and Z = 4. Cu(3-aPY)(4)NbOF5: monoclinic, space group P2(1)/n (No. 14), with a = 8.822(1) Angstrom, b 16.385(3) Angstrom, c = 8.902(1) Angstrom, beta = 109.270(3)degrees, and Z = 2.