Nuclear Magnetic Resonance and Optical Spectroscopy of [Co(MeOH)5Cl]+ in Methanol and the Kinetics of Methanol Exchange of the Solvation Shell

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Abstract
A study of the complex formation of Co+ + ions with Cl— ions in methanol at temperatures between —60° and —80°C is reported. The tools used are nuclear magnetic resonance and optical spectroscopy. It is shown that at these low temperatures the dominant cobalt—chlorine complex is an octahedral monochloro complex, [Co(MeOH)5Cl]+. The NMR spectra give the following values for the hyperfine interaction constants between methanol protons and Co+ +: 0.49, 0.33, and 2.4 Mc/sec for, respectively, the methyl protons of methanol cis to the chlorine, the methyl protons of methanol trans to the chlorine, and the hydroxyl proton in methanol cis to the chlorine. At temperatures above —75°C exchange of methanol between the coordination sphere and the bulk solvent contributes to the width of the NMR peaks. The results suggest that the dominant exchange process follows an SN1 mechanism, involving the dissociation of a cis methanol molecule in the complex as the rate‐determining step. The rate constant of this process is found to be 510 sec—1 at —63°C, and the activation energy 14.5 kcal/mole. Optical absorption spectra of the monochloro complex in the visible range are reported. The spectrum of this complex is similar to that of [Co(MeOH)6]+ +, but the maximum absorption is slightly shifted to longer wavelength (from 507 to 527 mμ at —80°C), and the extinction coefficient is 2.7 times that of [Co(MeOH)6]+ +.