The fact that we were able to obtain crystal structures for all six azophosphine-boranes 2-R enabled us to assess systematic trends as the electronic properties of the R group are varied (Table ). In general, the N1-N2 bond distance increases and the C9-N2 bond distance decreases as the R group becomes more electron-donating, as expected with the increased donation of electron density from the N-aryl substituent into the N=N π* orbital. This effect can be seen most clearly by comparing the two extreme azophosphine-boranes in terms of electronic properties, 2-NMe2 (C9-N2: 1.4047(18) Å; N1-N2: 1.2681(17) Å) and 2-CF3 (C9-N2: 1.443(2) Å; N1-N2: 1.235(2) Å). The percentage change in the P-N bond distances is much smaller across the series and not in a specific order, which is logical because the phosphine lone pair is forming a dative bond to the borane moiety and therefore cannot have a significant interaction with the neighbouring N=N π* orbital. The P1-N1-N2 bond angles also do not sequentially increase/decrease moving through the library, although there is a small degree of variation. The most significant differences in torsion angle within the series of 2-R are observed within 2-NMe2 and 2-CF3, which contain more than one crystallographically independent molecule, thus these differences can be assigned to crystal packing effects.