Anthony J. Downs

The molecular structures of the monomeric, pentacoordinated methylchloroniobium(IV) compounds Me₃NbCl₂and Me₂NbCl₃have 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 Me₂NbCl₃with the two axial NbCl bonds bentawayfrom the equatorial, slightly shorter NbCl bond. In the case of Me₃NbCl₂, moreover, the X‐ray model suggests structural distortions away from the idealizedC_3hgeometry, 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, inC_3vsymmetry, 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, inC_2vsymmetry, NbCl_ax2.361/2.304(5), NbCl_eq2.321/2.288(9), NbC 2.180/2.135(9), CH 〈1.094〉/1.12(1), ∢Cl_axNbCl_eq98.5/96.5(6), ∢CNbC 121.0/114(2), ∢NbCH 〈108.9〉/109(2). The electronic structures of Me₂NbCl₃and Me₃NbCl₂have 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.

The structures of gaseous tetrafluorohydridophosphorane, HPF₄, and trifluorodihydridophosphorane, H₂PF₃, have been determined by electron diffraction. The results indicate that both molecules adopt structures based on a trigonal bipyramid centred on the phosphorus atom, with the hydrogen atoms occupying equatorial sites, in conformity with C_2v symmetry. Salient structural parameters (r_a) are: (i) for HPF₄, r(P–F_eq) 153.7(0.4), r(P–F_ax) 159.1 (O.4), and r(P–H) 132.4(4.6) pm; H–P–F_eq 123.0(0.9) and H–P–F_ax 87.8(0.2)°; (ii) for H₂PF₃, r( P–F_aq) 154.4(0.4), r(P–F_ax) 161.3(0.2), and r( P–H) 132.2(1.1 ) pm; H–P–F_eq 119.6(4.4) and F_eq–P–F_ax 91.7(0.2)°. Replacement of fluorine by hydrogen thus results in unusually short P–H bonds, attenuation of the apical P–F bonds, and distortion of the PF_n fragment (n= 4 or 3) so that the apical P–F bonds are bent towards, while the equatorial P–F bonds in HPF₄ are bent away from, the hydrogen substituents.