Molecular structure of first-row transition metal dihalides from combined electron diffraction and vibrational spectroscopic analysis

The average and equilibrium molecular geometries and vibrational characteristics of MnCl2, FeCl2, CoCl2, NiCl2, MnBr2, FeBr2, CoBr2, and NiBr2 have been determined by a combined analysis of gas-phase electron diffraction and vibrational spectroscopic data. The nozzle temperatures ranged between 960 and 1100 K in the electron diffraction experiments, paralleled with mass spectrometric control of the vapor composition. Four approximations have been examined for the molecular Hamiltonian in the joint analysis. The overall utility of combined analysis has been demonstrated. The dynamic behavior of the first-row transition metal dihalides is best described by the semirigid model. The equilibrium bond distance is best approximated by the cubic anharmonic potential. The cubic force constants and the bond Morse anharmonicity parameter can be determined even from electron diffraction data alone provided that experimental information is collected to large enough scattering angles. The present analysis has confirmed the linearity of equilibrium configuration for all dihalides studied. The thermal average and equilibrium bond distances obtained are as follows, rg (re) with estimated total errors not exceeding ±0.007 Å, MnCl2 2.202 (2.184), FeCl2 2.151 (2.128), CoCl2 2.113 (2.090), NiCl2 2.076 (2.056), MnBr2 2.344 (2.328), FeBr2 2.294 (2.272), CoBr2 2.241 (2.223), and NiBr2 2.201 (2.177) Å. Appreciable amounts of dimeric species, with four-membered ring structures, were detected in the vapors of FeCl2, CoCl2, and FeBr2 under the electron diffraction experimental conditions. The largest amount, 11%, occurred for iron dibromide and a reliable geometry was determined for the Fe2Br4 molecule. The bridging metal–halogen bonds are about 0.2 Å longer than the terminal bonds, and the monomer bonds, in all three systems. The applicability of the joint diffraction/spectroscopic analysis is severely reduced in the presence of dimeric species.