Quantifying ligand steric properties

The space a ligand occupies around the metal center, or steric bulk, is an extremely important parameter for modulating the reactivity and stability of complexes. Ligand sterics are most often modified by changing the substituent groups of the donor atom. 

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Based on the experience with tertiary phosphines, the importance of the steric properties of NHC hgands in determining chemical behaviour has been immediately recognised. The main practical problem, however, is that NHC hgands substantiaUy present a local symmetry axis, whereas phosphines present a local symmetry axis. This imphes that the well-accepted molecular descriptor used to quantify steric properties in phosphines, the Tohnan cone angle [78], cannot be applied to NHC ligands. 

The concept of a structure-reactivity relationship implies that changes in structure should be quantitatively reflected in some measurable reactivity parameters associated with the molecule. For metal complexes, the capacity of the ligand structure to influence chemical properties is measurable in terms of reactivity parameters such as stability constants, rates of ligand dissociation, and reduction potentials. The influence of structure on chemical reactivity can often be rationalized in terms of steric and electronic components. MM calculations can be used to quantify the steric components present in the system. For metal complexes with series of ligands in which electronic effects are relatively constant (same number and type of donor atoms), reactivity differences can be attributed primarily to steric effects. In such cases, MM calculations have been used to obtain correlations between structure and reactivity. 

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