Group 6 Metal-Metal Bonds

This chapter describes recent developments in the chemistry of group 6 metal-metal (MM) quadruply and quintuply bonded compounds. The metal-metal quadruple bond is redox-active, and recent work describing its incorporation into functional materials and use in models for the study of electron transfer processes is discussed. More recently, the quadruple bond has been found to have fascinating photophysical properties. Studies elucidating the lifetimes associated with the relatively long-lived singlet and triplet excited states using transient absorption and time-resolved IR spectroscopies will be described. MM quintuply bonded compounds were first reported in 2005, and there are now around 20 structurally characterized examples employing chromium and molybdenum. Quintuply bonded compounds typically display supershort MM bond lengths, and the influence of the bridging ligand on the MM bond lengths is explored. The use of theoretical calculations to elucidate the electronic structure of quintuply bonded compounds is examined, which for chromium requires the use of multi-configurational theoretical calculations as there is often mixing of various bonding states at room temperature. The quintuple bond has also been found to react in a unique fashion with a number of small-molecules, and studies in this area will be discussed.