Molecules featuring metal-metal (M-M) bonds first received significant attention with regard to the nature of the intermetallic bonding, but have now grown into a vigorous field including applications as catalysts for organic transformations. There are also a number of cofactors of metalloenzymes that have two metals within range of forming M-M bonds (< 3 Å). For example, in hydrogenases, reduction of the cofactor places electrons into a M-M′ bond (M = Fe, M′ = Fe or Ni) that enables it to oxidatively insert the H2 substrate. In the FeMoco of molybdenum-dependent nitrogenase, the close metal-metal distances have fuelled speculation that M-M bonds could store electrons during the catalytic reduction of nitrogen. However, for iron systems (nitrogenases and others), there are typically bridging ligands, the effects of which are difficult to deconvolute from the M-M interaction itself.