W0318

Comparison of DFT Optimized Molecular Geometries Against the Golden Standard – X-Ray Structures. Ilia Guzei, 2124 Chemistry Dept., Univ. of Wisconsin-Madison, 1101 University Ave, Madison, WI 53706 USA.

Density functional theory is used to investigate the electronic and geometric structures and periodic trends in the Metal-Ligand bonding of the [O=NbCl₄-L]X compounds in which the metal complex is neutral or negatively charged and the coordinated atom of the ligand L is nitrogen, oxygen, or halogen. Crystal structures of anionic or neutral complexes of the formula [O=NbCl₄-L]^{(0 or –1)} where L is pyridine, acetonitrile, water, THF, halogen, or a vacant coordination site will be compared with the geometries of these complexes optimized at the UB3LYP/LANL2DZ level of theory.

Importantly, the theoretical model choice and its suitability to adequately describe a certain molecular system must be verified against the golden standard – X-ray structures, easily available to experimental crystallographers. Theoretical calculations coupled with X-ray analysis are a powerful source of information regarding electronic structures of complex compounds.

Trans-effects and 3-center 4-electron bonding in the Nb complexes [O=NbCl₄-L]^{(0 or –1)} will be addressed. A point of special interest is [O=NbCl₄(pyridine)]^–1, whose molecular geometry deviate from the ideal C_2v configuration in several structures. Its molecular structure, energy, and hybridization of the valent orbitals of the Nb center will be discussed.

The studies indicate that agreement between observed and calculated bond distances and angles acceptable to a theoretical chemist is significantly inferior to the one expected by a crystallographer. It is imperative to compare the experimental and theoretical molecular parameters to recognize the limitations of the model. However, theoretical calculations illustrate the expected trends in the bond distance and angle changes.