EXPERIMENTAL OBSERVATION OF THE LIGAND-OPPOSED CORE CHARGE CONCENTRATION IN VOCl₃, ACCOUNTING FOR ITS OBSERVED GEOMETRY WITH REGARD TO THE VSEPR MODEL

Christopher S Frampton¹, Moira-Ann Rennie^2, and Paul R. Raithby²

¹ Roche Discovery Welwyn, Broadwater Rd., Welwyn Garden City, Herts. AL7 3AY, UK.
² Department of Chemistry, University of Cambridge, Lensfield Rd., Cambridge CB2 1EW, UK.

Keywords: Charge Density, VOCl₃, Laplacian, VSEPR.

The Laplacian of the electronic charge distribution, the quantity (d²r), confirms the presence of local concentrations and depletions of charge in the valence shell of an atom within a molecule.[1] The local maxima exactly duplicate in number, spatial location and size the electron pairs of the Valence Shell Electron Pair Repulsion, (VSEPR), model.[2] The VSEPR model, however, often fails to predict the observed geometry in cases when the central atom, A, of an AX_n system is a transition metal, for example compare POCl₃ and VOCl₃.

Using Ab-Initio RHF SCF calculations Bader and co-workers have proposed an extension of the applicability of the VSEPR model such that the outer shell of core electrons of the central metal atom, A, are perturbed by the polarising field of the ligands, B, to produce localised concentrations of charge spatially opposed to the metal-ligand bond. These charge concentrations are known as Ligand-Opposed Core Charge Concentrations, (LOCCC's).[2,3,]

A high resolution X-ray charge density study of VOCl₃ at 123 K was undertaken to enable the experimental observation of the LOCCC's which have so far only been predicted from theoretical calculations. VOCl₃ was chosen since its crystal structure was known, [4], and a detailed RHF SCF theoretical calculation was available.[2] The experimental task was, however, made more daunting by the fact that VOCl₃ is a highly corrosive liquid and a crystal had to be grown in-situ before the charge density data collection could begin.

This presentation will reveal details of the charge density experiment including crystal growth, high resolution data collection on a SMART CCD diffractometer, data reduction, the final multipole model adopted and experimental maps showing the LOCCC's. We believe this to be the first experimental observation of a LOCCC.

[1] R.F.W. Bader, (1990), Atoms in Molecules, OUP, Oxford, England.
[2] P.J. MacDougall et al, (1989), Can. J. Chem., 67, 1842-1846.
[3] R.J. Gillespie et al, (1996), Inorg. Chem., 35, 3954-3963 and references therein.
[4] J. Galy et al, (1983), J. Solid State Chem., 47, 143-150.