Gas-phase structure of nickel dichloride. An electron-diffraction investigation augmented by ab initio and DFT calculations

Abstract

The structure of nickel dichloride in the vapor at ca. 1050 K has been studied experimentally by electron diffraction and theoretically by ab initio and DFT molecular orbital calculations. The molecule is found to have a linear equilibrium structure, and from all levels of theory it is predicted to have a triplet ground state. Theory also predicts the existence of a singlet state about 30 kcal/mol less stable than the triplet where the molecule is triangular with a bond angle in the range of about 110°̊ – 117°̊. Two models of the experimental structure were investigated. Model A consisted of 11 “pseudoconformers” distributed at even intervals over the range 180°̊ ≤ ∠(ClNiCl) ≤ 130°̊ for which the main refined parameter was the distribution (relative weighting) of the pseudoconformers based on an assumed bending potential of the form V(Δθ) = V2(Δθ)2, where Δθ is the difference of the ClNiCl angle from linearity. In this model the pseudoconformational weighting led to averages for the bond- and nonbond distances. Model B was simpler, defined only by values of the bond distance and bond angle. The structural results for Model A are 〈rg(NiCl)〉 = 2.067(6) Å, 〈rg(Cl⋅⋅Cl)〉 = 4.037(28) Å, 〈lg(NiCl)〉 = 0.081(6) Å, and 〈lg(Cl⋅⋅Cl)〉 = 0.136(52) Å; in this case the rms amplitudes are for the frame (no contributions from the bending mode). For Model B they are 〈rg(NiCl)〉 = 2.059(5) Å, 〈rg(Cl⋅⋅Cl)〉 = 4.041(89) Å, 〈lg(NiCl)〉 = 0.081(6) Å, and 〈lg(Cl⋅⋅Cl)〉 = 0.159(60) Å; the uncertainties are estimated 2σ. The theoretical prediction of a linear ground state for nickel dichloride is confirmed by experimental values of about 156̊° for the bond angle ∠(ClNiCl) in each model, which differs from the theoretical 180̊° due to the effects of vibrational averaging that arise nearly entirely from the bending mode. The structure is discussed.