Cobalt(II) chloro complexation in N-methylformamide-N, N-dimethylformamide mixtures: TTTTitration calorimetry and spectrophotometry

Cobalt(II) chloro complexation has been studied by titration calorimetry and spectrophotometry in solvent mixtures of N-methylformamide (NMF) and N,N-dimethylformamide (DMF). It revealed that a series of mononuclear CoCln _n ^(2-n)+ (n=1-4) complexes are formed in the mixtures of NMF mole fraction x _NMF=0.05 and 0.25, and the CoCl⁺, CoCl₃ ^- and CoCl₄ ^2- complexes in the mixture of x _NMF=0.5, and their formation constants, enthalpies and entropies were obtained. As compared with DMF, the complexation is markedly suppressed in the mixtures, as well as in NMF. The decreasing formation constant of CoCl⁺ with the NMF content is mainly ascribed to the decreasing formation entropy. DMF is aprotic and thus less-structured, whereas NMF is protic to form hydrogen- bonded clusters. In DMF-NMF mixtures, solvent clusters in neat NMF are ruptured to yield new clusters involving DMF, the structure of which depends on the solvent composition. The entropy of formation of CoCl ⁺ will be discussed in relation to the liquid structure of DMF, NMF and their mixtures.