Metal-phosphine bonding revisited. σ-Basicity, π-acidity, and the role of phosphorus d orbitals in zerovalent metal-phosphine complexes by Gianfranco Pacchioni and Paul S. Bagus (1992, 10.1021/ic00047a029)

Inorganic Chemistry 31, 4391–4398. Orbital calculations separate sigma basicity and pi acidity; this is a theoretical bonding study, not a measured universal spectrochemical ranking.