Ultrashort metal-metal distances and extreme bond orders.

Chemical bonding is at the very heart of chemistry. Although main-group-element E-E' bond orders range up to triple bonds, higher formal bond orders are known between transition metals. Here we review recent developments related to the synthesis of formally quintuply bonded transition metals in coordination compounds, and their theoretical description. The quadruple bond fascinated chemists for about 40 years. Recently, a stable molecule containing a formal quintuple bond initiated a renaissance in synthesizing and understanding bonds with high bond orders. Ultrashort metal-metal distances as low as 1.73 Å are one of the outcomes. First results indicate that the relevance of these bimetallic platforms to synthetic chemistry can be addressed through quintuple-bond reactivity studies. The theoretical description of the bonding situation in molecules with extreme bond orders has only just begun.