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Steric Parameters An organometallic complex is dynamic, with ligands and their substituents accessing different conformations within 3 - 5 kcal/mol (depending on experimental temperature) of the ground state geometry. There are two different approaches taken within the research to quantify this conformational flexibility.
Molecular Mechanics is used to rotate two bonds that significantly alter the size of a ligand. The energy surface (a) is used to enforce a cut-off for inaccessible geometries. Then a second plot (b) is generated for a steric measure that describes the size of the ligand, with only energetically accessible conformations shown. In the example to the right a phosphine is calculated, so the cone angle steric measure was used to describe the ligand size. To quantify the apparent restricted rotation of large phosphine ligands (L), methodically scanning the rotation of the metal-phosphorus bond provides an energetic barrier, the peak of which coincides with the largest substituents passing / clashing - representing the steric impact on the complex as the ligand is rotated about the metal-donor bond – and the likelihood of dynamic rotation in solution. Publications:
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