Early work on the kinetics of first transition series metal ions suggested that the mechanism for six coordinate ions was dissociative in nature. This was contrasted with the well established associative reactions of four coordinate metal ions. Recent work has shown that while the dissociative mechanism applies to 2+ metal ions late in the transition series, Co²⁺ and Ni²⁺ for example, metals early in the series, such as V²⁺ and Mn²⁺, react by an associative mechanism. The mechanistic type of these reactions has been established by measurements of volumes of activation. High pressure NMR data has been very useful in this work. Molecular orbital calculations have been helpful in interpreting the results.