- 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 degrees - 117 degrees. Two models of the experimental structure were investigated. Model A consisted of 11 "pseudoconformers" distributed at even intervals over the range 180 degrees <= (sic)(CINiCI) <= 130 degrees for which the main refined parameter was the distribution (relative weighting) of the pseudoconformers based on an assumed bending potential of the form V(Delta theta) = V-2(Delta theta)(2), where Delta theta is the difference of the CI-Ni-CI 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(Ni-CI)) = 2.067(6) angstrom, (r(g)(Cl center dot center dot center dot CI)) = 4.037(28) angstrom, < l(g)(Ni-CI)) = 0.081(6) angstrom, and.Cl)) = 0.136(52) angstrom; in this case the rms amplitudes are for the frame (no contributions from the bending mode). For Model B they are < r(g)(Ni-Cl)) = 2.059(5) angstrom, < r(g)(Cl center dot center dot Cl)> = 4.041(89) angstrom, < l(g)(Cl center dot center dot center dot Cl)> = 0.081(6) angstrom, and < I-g(Cl center dot center dot Cl)> = 0.159(60) angstrom; the uncertainties are estimated 2 sigma. The theoretical prediction of a linear ground state for nickel dichloride is confirmed by experimental values of about 156 for the bond angle (sic)(Cl-Ni-Cl) in each model, which differs from the theoretical 180 due to the effects of vibrational averaging that arise nearly entirely from the bending mode. The structure is discussed. (C) 2018 Published by Elsevier B.V.