Chapter One - Ligand Steric Descriptors

Abstract

Abstract Descriptors for the steric size of transition metal-bound ligands suffer from coarse approximations and complicated correction schemes. Common methods such as the Tolman cone angle and solid angle set a universal value for each ligand regardless of the effect of steric environment on ligand geometry. We have developed two approaches to quantify sterics that improve upon the previous descriptors: the exact cone angle (θ°) and the exact solid angle (Θ°). The exact cone angle encapsulates a ligand inside of the smallest possible right circular cone without making any approximations concerning metal–ligand bond length or ligand geometry. The exact solid angle quantifies the area screened by a ligand on the surface of a surrounding sphere centred at the metal atom. While the cone angle only describes a single ligand, the solid angle accounts for the sterics of an entire complex. Herein, the mathematics behind each is described and numerous examples are given. An intriguing case models the change in sterics of first- and second-generation Grubbs’ catalysts over the course of a ligand dissociation reaction.