Arne Haaland

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 molecular structures of dimethylamino[(dimethylboryl)methylamino]methylborane, Me₂NBMeNMeBMe₂ (1) and 1,1-bis(dimethylboryl)-2,2-dimethylhydrazine, (Me₂B)₂NNMe₂ (2) have been determined by gas electron diffraction (GED), density functional theory calculations at the B3PW91/6-311++G** level and ab initio calculations at the MP2/6-311++G** level. 1 adopts an open structure similar to that of the isoelectronic hydrocarbon molecule permethylbutadiene; the central B–N bond distance at 148.0/149.3(7) pm (MP2/GED) corresponds to a single covalent N–B bond distance, the two terminal distances, 140.9/140.5(4) pm and 141.8/141.3(4) pm, correspond to the distance between N and B atoms joined by a covalent σ-bond and a dative π-bond. A closed form where the establishment of a dative bond between the terminal N and B atoms has led to the formation of a four-membered ring also corresponds to a minimum on the potential energy surface, but the energy is calculated to be 14.3 kJ mol⁻¹ higher at the MP2 level. This structure is also incompatible with the GED data. 2 adopts a structure in which a dative σ-bond between the dimethylamino N atom and one of the boron atoms has led to the formation of a three-membered N₂B ring. The dative σ-bond distance is 165.5/164.0(13) pm, the two other bond distances in the ring are N–B = 150.6/148.9(9) pm corresponding to a covalent σ-bond and N–N = 145.1/145.4(3) pm. The terminal B–N distance 139.6/138.9(9) pm is consistent with a covalent σ-bond augmented by a dative π-bond. An open Y-shaped structure also corresponds to a minimum on the potential energy surface, but the energy is 18.7 kJ mol⁻¹ higher (MP2) and it is incompatible with the GED data.

Contrary to the published acknowledgement, Professor R. F. W. Bader in no way contributed to any of arguments presented in this paper; he has indicated that all of the criticisms raised by the authors were previously discussed and rebuted in J. Chem. Phys. A 1998, 102, 7314.A copy of an earlier version of the manuscript was sent to Professor Bader last spring, and the e‐mail correspondence that followed made it very clear that he disagreed strongly with the conclusions. The authors nevertheless found the discussion useful, since it clarified the nature of the disagreement. Professor Bader's queries also prompted the authors to calculate the dissociation energy of the He@adam inclusion complex and to determine the structure of the transition state as discussed in the article. The authors apologize for omitting to ask for Professor Bader's permission to include his name among the acknowledgements.

The structure, energetics, and electron density in the inclusion complex of He in adamantane, C₁₀H₁₆, have been studied by density functional theory calculations at the B3LYP6‐311++G(2p,2d) level. Topological analysis of the electron density shows that the He atom is connected to the four tertiary^tC atoms in the cage by atomic interaction lines with (3,−1) critical points. The calculated dissociation energy of the complex He@adamantane(g)=adamantane(g) + He(g) of ΔE=−645 kJ mol⁻¹nevertheless shows that the He–^tC interactions areantibonding.

Density functional theory calculations on E(SbMe₂)₂, E = S or Te, Me = CH₃, indicate that the equilibrium structures are syn-syn or near syn-syn conformers with overall C_2v or C₂ symmetry. The calculations further indicate the existence of syn-anti conformers about 4 kJ mol^-1 (E = S) or 1 kJ m ol^-1 (E = Te) above the equilibrium structure. G as electron diffraction data show that both conform ers are present in gaseous S(SbMe₂)₂, while the presence of the syn-anti conformer in gaseous Te(SbMe₂)₂ is uncertain. The Sb - S and Sb - Te bond distances are 241.4(4) and 278.1(3) pm, respectively, the valence angles of the synsyn conformers are <SbSSb = 9 8.7(5)° and <SbTeSb = 91(2)°.

The gas electron diffraction pattern of monomeric UCl₄ has been recorded with a nozzle temperature of 900 K. The data are in good agreement with a model of T_d symmetry and a U–Cl bond distance of r_a= 250.3(3) pm. The root-mean-square vibrational amplitudes are l(U–Cl)= 8.9(3) pm and l(Cl ⋯ Cl)= 34.3(10) pm. The gas-phase infrared absorption spectra have been recorded from 25 to 3400 cm^–1 at temperatures ranging from 700 to 900 K, and the t₂ stretching and deformation modes have been assigned at ν₃ 337.4 and ν₄ 71.7 cm^–1 respectively. The molecular structure of UCl₄ has been optimised by density-functional calculations under D_2d symmetry. It is tetrahedral with bond distance r_e= 251 pm. The calculations yield values for the IR-active modes which are in good agreement with the experimental counterparts. The ‘best values’ for the two IR-inactive frequencies were obtained by refining a diagonal symmetry force field to the four calculated (density functional) frequencies, the two observed frequencies and the vibrational amplitudes obtained by gas electron diffraction. The entropies of gaseous UCl₄ calculated from these normal modes and a symmetry number of 12 corresponding to T_d symmetry are in good agreement with the experimental counterparts.