( 1 JP-Se) of the corresponding azophosphine selenides (3-R; Scheme 2) by 31 P{ 1 H} NMR spectroscopy ( 77 Se: 7.6% abundant, I = ½). This approach is well-established in the literature as an alternative to the more traditional measurement of the Tolman electronic parameter. Providing the steric bulk surrounding the phosphorus atom remains consistent, the value of the 1 JP-Se coupling constant is dependent on the electronic nature of the substituents at P and, as governed by Bent's rule and the Fermi-contact interactions of the s-orbitals of P and Se, increases with increasingly electron-withdrawing substituents. The magnitude of 1 JP-Se therefore inversely corresponds to the relative σ-donor capability of a homologous series of P-based ligands.