Valence Shell Charge Concentrations at Pentacoordinate d0 Transition-Metal Centers: Non-VSEPR Structures of Me2NbCl3 and Me3NbCl2

• Published in: Chemistry : a European journal 2005-08, Vol.11 (17), p.4921-4934
• Main Authors: McGrady, G. Sean, Haaland, Arne, Verne, Hans Peter, Volden, Hans Vidar, Downs, Anthony J., Shorokhov, Dmitry, Eickerling, Georg, Scherer, Wolfgang
• Format: Article
• Language: English
• Subjects: charge density; gas electron diffraction; niobium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200400808

The molecular structures of the monomeric, pentacoordinated methylchloroniobium(IV) compounds Me3NbCl2 and Me2NbCl3 have been determined by gas electron diffraction (GED) and density functional theory (DFT) calculations, and, for Me3NbCl2, by single crystal X‐ray diffraction. Each of the molecules is found to have a heavy‐atom skeleton in the form of a trigonal bipyramid (TBP) with Cl atoms in the axial positions, in accord with their vibrational spectra. The TBP is somewhat distorted in the case of Me2NbCl3 with the two axial NbCl bonds bent away from the equatorial, slightly shorter NbCl bond. In the case of Me3NbCl2, moreover, the X‐ray model suggests structural distortions away from the idealized C3h geometry, in line with the results of quantum chemical calculations. Structure optimizations by DFT calculations and least‐squares refinement to the GED data yield the following structural parameters (calcd/exptl; eq=equatorial; ax=axial; distances in Å, angles in degrees; average values in 〈〉 brackets): Me3NbCl2, in C3v symmetry, NbCl 〈2.370〉/〈2.319(3)〉, NbC 2.173/2.152(4), CH 〈1.096〉/1.124(5), ∢NbCH 〈109.3〉/105.2(8), ∢ClNbC 92.2/93.3(2), ∢CNbC 119.9/119.7(1); Me2NbCl3, in C2v symmetry, NbClax 2.361/2.304(5), NbCleq 2.321/2.288(9), NbC 2.180/2.135(9), CH 〈1.094〉/1.12(1), ∢ClaxNbCleq 98.5/96.5(6), ∢CNbC 121.0/114(2), ∢NbCH 〈108.9〉/109(2). The electronic structures of Me2NbCl3 and Me3NbCl2 have been explored by rigorous analysis of both the wavefunction and the topology of the electron density, employing DFT calculations. Hence the structures of these compounds are shown to reflect repulsion between the NbC and NbCl bonding electron density and charge concentrations induced by the methyl ligands in the valence shell of the Nb atom and arising mainly from use of Nb(4d) functions in the NbC bonds. Ligand‐induced charge concentrations (LICCs) are shown to cause the non‐VSEPR structures, determined by experiment and calculation, for the heteroleptic d0 transition‐metal alkyls Me3NbCl2 and Me2NbCl3 (see figure). The natural bond order (NBO) method was employed to partition the total charge density in a physically meaningful way, and we demonstrate for the first time that all information about the origin of LICCs in d0 transition‐metal compounds is directly accessible through the wavefunction. LICCs arise naturally as part of an MX bond in which metal d‐functions are involved; also a sound chemical and physical basis is outlined for their occurrence