Recent advances in the chemistry of metal–metal quadruple bonds

Metal–metal quadruple bonds can be formed between d⁴ metal ions, and tend to adopt one of two geometries; square parallelpiped or paddlewheel. In this chapter we outline recent efforts in the synthesis and characterization of quadruply bonded complexes of Cr(ii), Mo(ii), W(ii), Te(iii) and Re(iii). The quadruple bond is redox active and has a unique electronic structure, which results in it having interesting reactivity and photophysical properties. Their successful application as strong reducing agents, and in the study of electron transfer processes in the ground and photoexcited states, will be discussed.