Quantifying and understanding the steric properties of N-heterocyclic carbenes

This Feature Article presents and discusses the use of different methods to quantify and explore the steric impact of N-heterocyclic carbene (NHC) ligands. These include a) the percent buried volume (%Vbur), which provides a convenient single number to measure steric impact and b) steric maps, which provide a graphical representation of the steric topology of a ligand using colour-coded contour maps. A critical discussion of the scope and limitations of these tools is presented, along with some examples of their use in organometallic chemistry and catalysis. This Article should provide all users of NHCs ( and of other ligand families) from organic, organometallic, and inorganic chemistry backgrounds, with an appreciation of how these tools can be used to quantify and compare ligand steric properties.