Characterization of lone electron pair using Liebau density vector and Wang-Liebau eccentricity parameters

MURSHED M.M.; MENDIVE C.; CURTI M.; GESING TH.

Basel

Conferencia; The 30th European Crystallographic Meeting (ECM-30); 2016

European Crystallographic Association

Compounds carrying cations with lone electron pair (LEP)show marked distortion in their coordination sphere, which is intimatelyrelated to the electronic distribution of the LEP. Hence, non-spherical butasymmetric LEPs are usually referred to be stereochemically active. It is stillin search of a measure to quantify at which extend the LEP of a cation in acrystal structure is stereochemically active. Herein we approach the Wang-Liebau eccentricity (WLE) parameter [1] and the Liebau densityvector (LDV) [2] to characterize the 6s2 LEP of Bi3+ cations in Bi4B2O9as a model compound. WLE is a vector term associated with all bonds in a givencoordination, which measures the electronic deformation density of the LEP anddirects along the line from the nucleus to the approximate center of the LEP chargedensity. The dimensionless length of the WLE parameter thus indirectly measuresthe degree of deformation of the associated LEP, and cannot be depictedgraphically. LDV describes the length of a vector directed from the nucleus ofan atom to the maximum electron density of the LEP. We have demonstrated onsome mullite-type compounds that the direction of WLE and LDV are almostparallel as long as a localized electron density is obtained for LDV [2]. Theambient condition crystal structure of Bi4B2O9(P21/c) [3,4] possesses four crystallographically differentBi-positions, where Bi1 and Bi3 are six-coordinated and, Bi2 and Bi4 are sevenand eight coordinated, respectively, within Bi-O distance less than 330 pm. Boththe WLE and LDV magnitudes for Bi1 and Bi3 are higher than those of the highercoordinated Bi2 and Bi4. A correlation between the LDV and the electron densityof LEP determines the minimum localized electron density of LEP to be stereochemicallyactive irrespective to the crystallographic position of the Bi-atom. Withincreasing temperature both the WLE and LDV decreased, indicating the decreaseof stereochemical activity of the LEP. References:[1] X. Wang, F. Liebau, Z. Kristallogr. 211 (1996) 437.[2] M. Curti, Th.M. Gesing, M.M. Murshed, T. Bredow, C.B.Mendive, Z. Kristallogr. 228 (2013) 629.[3] S.K. Filatov, Y.F.Shepelev, Y.V. Aleksandrova, R.S. Bubnova, Russ. J. Inorg. Chem. 52 (2007) 21.[4] A.Hyman, A. Perloff,  Acta Crystallogr. 28 (1972) 2007.