Infrared spectra of the matrix-isolated chlorides of iron,cobalt, and nickel

Iron, cobalt, and nickel metal cathodes were sputtered with various mixtures of ³⁷Cl₂/³⁵Cl₂/Ar to produce Ar matrices at 14 K that contained the mono-, di-, and trichlorides of the corresponding metal. The measured infrared absorption spectra of these matrices allowed the identification and characterization of FeCl₂, FeCl₃, CoCl, CoCl₂, CoCl₃, and NiCl₂. The derived vibrational constants of the electronic ground state of ⁵⁹Co³⁵Cl are ω_e = 457.8 ± 3.0 cm^?1 and ω_eχ_e = 2.0 ± 1.5 cm^?1. The antisymmetric stretching mode frequencies of six isotopomers of FeCl₂ and three isotopomers of CoCl₂ were identified and measured. The dichlorides of iron, cobalt, and nickel were all determined, in contrast with previous work, to be nonlinear with bond angles of 161, 157, and 161°, respectively. The estimated uncertainty is 5°. For both iron and cobalt trichlorides, the measured stretching mode frequencies were used to derive a ClMCl angle in excess of 120°, as would be expected for planar molecules with somewhat anharmonic vibrations. Observed adsorption peaks could be assigned to the ν₃(E) modes of planar (D_3h) Fe and Co trichlorides and the corresponding modes of the isotopomers. These observations strongly suggest that recent data supporting a pyramidal geometry for FeCl₃ should be reexamined. The geometry of CoCl₃ has not previously been determined.