Charge density studies in NH4[Ti(C2O4)2].2H2O crystals at two wavelengths

Abstract

The crystal structure of ammonium bis-μ-oxalatotitanate(III) dihydrate, NH4[Ti(C2O4)2].2H2O, is reinvestigated by careful single-crystal X-ray diffraction using Mo Kα (room temperature and 130 K) and Ag Kα radiation (room temperature). It crystallizes in noncentrosymmetric hexagonal space group P6422, with cell parameters a = 8.947 (2), c = 10.898 (1) Å, Z = 3, F(000) = 423, Mr = 278.1, Dx = 1.833, R = 0.035, wR = 0.030 for 2296 observed reflections with I ≥ 2σ(I), on Ag Kα data. Accurate data measurement was applied using two wavelengths (0.7107 Å for Mo Kα and 0.5609 Å for Ag Kα in order to study the charge density distribution and also to investigate the wavelength effect in such investigations. A total of 22650 and 20298 reflections were measured using Mo Kα radiation at room temperature and 130 K, respectively, and 18361 reflections using Ag Kα radiation. The Ti atom is coordinated by four oxalate dianions with coordination number 8 in an approximate symmetry of D 4 geometry. Each oxalato ligand is bridged between two Ti atoms and forms an infinite polymeric spiral column along the c axis. The deformation density maps, Δρ, obtained from all three data sets are consistent and agree well. Although the formal charge of Ti in this complex is 3+, with only one electron on the 3d orbitals expected, the net atomic charge on Ti from this study is, ~2+. Charge asphericity around the Ti atom caused by the splitting of 3d orbitals is clearly observed in the deformation density. The d-orbital populations are evaluated from multipole refinement. The expected d 1 electron is mainly located in the dZ2 orbital.