Evidence for nonpolar alignment of [NbOF₅]^2- anions in Cd(pyridine)₄NbOF₅ chains

A new phase of Cd(py)₄NbOF₅ (py = pyridine) (A) is a rare example of a mixed metal oxide fluoride in an ordered linear chain structure. This phase is of particular interest because it is a polymorph of a previously reported Cd(py)₄NbOF₅ phase (B) that crystallizes in disordered linear chains. The ordered polymorph A contains two distinct Cd²⁺ sites: one which is coordinated to the oxide ligands from two [NbOF₅]^2- units, and another that is coordinated to two trans fluoride ligands. As a result, the structure of polymorph A features nonpolar linear chains with locally ordered [NbOF₅] ^2- anions. This ordering is explained in terms of the inherent electronic differences between oxide and fluoride ligands. The stabilization provided by π-stacking interactions between pyridine ligands on neighboring chains, as well as the effects of reactant stoichiometry on the general structure of the products in the CdO·1/2Nb₂O ₅/(HF)_x·py/H₂O system, is also discussed. Crystal data for the ordered Cd(py)₄NbOF₅ polymorph: monoclinic space group P2₁/c (No. 14) with a = 14.097(1) Å, b = 16.353(1) Å, c = 10.4380(9) Å, β = 95.569(2)°, and Z = 4.