Which phosphines squeeze together? Phosphine ligands coordinated to palladium and nickel are essential tools for assembling the backbones of pharmaceutical compounds. For decades, descriptors that characterize spatial bulk have helped to guide phosphine optimization. However, these descriptors tend to apply to ideal geometries of a single ligand. Newman-Stonebraker et al . introduce a descriptor that considers how the ligand conformation might change in a crowded environment. Specifically, they found that the minimum percentage buried volume accurately predicts when one or two of a particular ligand will coordinate to a metal center, frequently a key determinant of successful catalysis. —JSY