CHARGE-DENSITY STUDIES IN NH4[TI(C2O4)(2)].2H(2)O CRYSTALS AT 2 WAVELENGTHS

The crystal structure of ammonium bis-mu-oxalato-tinanate(III) dihydra te, NH4[Ti(C2O4)(2)].2H(2)O, is reinvestigated by careful single-cryst al X-ray diffraction using Mo K alpha (room temperature and 130 K) and Ag K alpha radiation (room temperature). It crystallized in noncentro symmetric hexagonal space group P6(4)22, with cell parameters a = 8.94 7 (2), c = 10.898 (1) Angstrom, Z = 3, F(000) = 423, M(r) = 278.1, D-x = 1.833, R = 0.035, wR = 0.030 for 2296 observed relections with I gr eater than or equal to 2 sigma(I), on Ag K alpha data. Accurate data m easurement was applied using two wavelengths (0.7107 Angstrom for Mo K alpha and 0.5609 Angstrom for Ag K alpha) in order to study the charg e density distribution and also to investigate the wavelength effect i n such investigations. A total of 22 650 and 20 298 reflections were m easured using Mo K alpha radiation. The Ti atom is coordinated by four oxalate dianions with coordination number 8 in an approximate symmetr y of D-4 geometry. Each oxalato ligand is bridged between two Ti atoms and forms and infinite polymeric spiral column along the c axis. The deformation density maps, Delta rho, obtained from all three data sets are consistent and agree well. Although the formal charge of Ti in th is complex is 3+, with only one electron on the 3d orbitals expected, the net atomic charge on Ti from this study is similar to 2+. Charge a sphericity around the Ti atom caused by the spitting of 3d orbitals is clearly observed in the deformation density. The d-orbital population are evaluated from multipole refinement. The expected d(1) electron i s mainly located in the d(z2) orbital.