To explore the electronic structure of azophosphines 1-R in more detail, density functional theory (DFT) calculations and natural bond orbital (NBO) analyses were carried out (see SI for details). The N-N bond distances in this series are all in a very narrow range of 1.236-1.239 Å (values in Table ), which implies there is an approximately equal degree of delocalisation of electron density into the N=N π* orbital across the series. However, the source of this donated electron density varies as a function of the nature of the R group, as shown by the two resonance structures α and β (Figure ). Natural population analysis (NPA) shows an increasingly electron-poor phosphorus centre as more electronwithdrawing substituents are used. This is consistent with a higher contribution from resonance α as the phosphorus lone pair is increasingly delocalised into the N=N π* orbital and extended π-system. Conversely, with more electron-donating substituents, the NPA value for N1 becomes more negative, consistent with a higher contribution from resonance β as more electron density resides on the formally anionic N1 centre. The NPA values are further supported by examination of the donor/acceptor interactions, which show larger donations from the neighbouring aryl π orbital into the N=N π* orbital with electron-donating substituents, and increasing donation of the phosphorus lone pair into the N=N π* orbital withelectron-withdrawing substituents (see Table for all values). The similarity of the lengths of the N=N bonds across the series is reflective of both resonance forms α and β imparting similar effects on the bond. These data underline the effects of both competing resonances on azophosphines, and suggest that control over substituent may have an important role on the applications of azophosphines as ligands, with electron-donating substituents yielding a more electron-rich phosphorus centre.