Abstract Metal NHC complexes have been known for over 50 years, but only during the last 15 have they been studied as potential catalysts. N-heterocyclic carbene (NHC) complexes are often more straightforward to make than the phosphine analogues with which they are often compared, are generally more stable, less odorous and on many occasions have been shown to be anywhere between 100 and 1000 times more effective. For a decade it was assumed that NHCs always interacted with metal atoms in the same way. However, examples have now been found that exhibit unusual binding behaviour. It is important to understand why such abnormal binding occurs and what effects this could have on the catalytic properties of NHC complexes. Download : Download full-size image