Vibrational spectroscopy of Co⁺(CH₄)n and Ni⁺(CH₄)n (n = 1-4)

Vibrational spectra of M(+)(CH4)m(Ar)(3-m) and M(+)(CH4)n (M = Co, Ni; m = 1, 2; n = 3, 4) in the C-H stretching region (2500-3100 cm(-1)) are measured using photofragment spectroscopy, monitoring the loss of argon or methane. Interaction with the metal leads to large red shifts in the C-H stretches for proximate hydrogens. The extent of this shift is sensitive to the coordination (η(2) vs η(3)) and to the metal-methane distance. The structures of the complexes are determined by comparing measured spectra with those calculated for candidate structures at the B3LYP/6-311++G(3df,3pd) level. Binding energies are also computed using the CAM-B3LYP functional. In all cases, CH4 shows η(2) coordination to the metal. The m = 1 complexes show very large red shifts of 370 cm(-1) (for M = Co) and 320 cm(-1) (for M = Ni) in the lowest C-H stretch, relative to the symmetric stretch of free CH4. They adopt a C2v structure with the heavy atoms and proximate hydrogen atoms coplanar. The m = 2 complexes have slightly reduced red shifts, and Tee-shaped structures. Both Tee-shaped and equilateral (or quasi-equilateral) structures are observed for the n = 3 complexes. The measured photodissociation onset and significantly reduced intensity for low-frequency C-H stretches imply a value of 2650 ± 50 cm(-1) for the binding energy of Ni(+)(CH4)2-CH4. The Co(+)(CH4)4 complexes have two low-lying structures, quasi-tetrahedral and distorted square-planar, which contribute to the rich spectrum. In contrast, the symmetrical, square-planar Ni(+)(CH4)4 complex is characterized by a very simple vibrational spectrum.