Rollover cyclometalation - early history, recent developments, mechanistic insights and application aspects

Rollover cyclometalation constitutes a special case among the well-known class of cyclometalation reactions. An overview is given that covers the very first description of this reaction type, as well as recent developments. In addition, not only condensed-phase experiments are reviewed, but also investigations based on mass spectrometric techniques, together with in silico studies using DFT-based calculations are considered. While the latter two methods allow for a detailed analysis of the intrinsic factors that affect the reaction mechanisms, consideration of all three regimes permits to develop a coherent mechanistic picture and to address the often noted gap between condensed-and gas-phase studies. Moreover, the quite unexpected reactivity of rollover cyclometalated complexes in gas-phase experiments, as well as potential applications, e. g. in synthetic procedures, are discussed in some detail.