We previously reported the exclusive κ 1 P coordination of MesN2P t Bu2 to a chloro-Ru(pcymene) framework, and how κ 2 P,N coordination could be achieved by using an azophosphine with a smaller steric profile at the ortho-positions flanking the terminal nitrogen (1-NMe2) and switching the arene on Ru to benzene. Continuing our study of the coordination chemistry of azophosphines, we were keen to learn whether fine-tuning of their electronics, in addition to sterics, could also affect κ 2 P,N coordination. (5-OMe) (Figure ). Pleasingly, this demonstrated that κ 2 P,N coordination of 1-OMe was compatible with the bulkier η 6 -(p-cymene) on the Ru centre, and did not require the switch to the η 6 -benzene moiety. Scheme 3. General synthesis used to evaluate κ 2 P,N coordinative ability of azophosphines. ), N1-N2 1.283(2), P1-N1-N2 99.18 (11), P1-Ru1-N2 62.92(4). Thermal ellipsoids were drawn at the 50% probability level. To test how modulating the electronics of 1-R affected the ability of the azophosphine to act as a competent κ 2 P,N ligand, we probed the formation of the bidentate complex with the increasingly electronpoor azophosphines. To chlorobenzene solutions of each azophosphine 1-R (R = OMe, Me, H, F, CF3) was added half an equivalent of the [Ru(pcymene)Cl2] dimer. Without isolation of the corresponding Ru(p-cymene)(κ 1 P-(1-R))Cl2 (4-R) products, NaBPh4 was added to initiate κ 2 P,N coordination to attempt to access the respective complex [Ru(p-cymene)(κ 2 P,N-(1-R))Cl][BPh4] salts (5-R), as assessed by P{ 1 H} NMR spectroscopy without any further purification (Figure ). These experiments show that the electron-rich azophosphine 1-OMe affords 5-OMe in quantitative yields under these conditions (as measured by P{ 1 H} NMR spectroscopy). 1-Me and 1-H formed predominantly 5-Me (71.5 ppm) and 5-H (73.6 ppm), respectively, however the crude spectra included trace signals for 1-Me (113.4 ppm), 1-H (115.1 ppm), 4-Me (116.5 ppm), and a trace unknown impurity (R = Me, 130.1 ppm; R = H 131.6) (Figure ). 1-F also formed mostly 5-F (74.3 ppm), however, the crude spectrum showed signals associated with 1-F (115.4 ppm) and an unknown impurity (133.3 ppm) of greater intensity than when R = Me or H. 1-CF3 was shown not be an effective ligand for this transformation. It is possible that a small amount of 5-CF3 was formed (tentatively assigned as 72.9 ppm), but the crude spectrum showed two main signals at 59.8 ppm and 112.1 ppm, and a smaller signal at 165.7 ppm; none of which can be assigned to 1-CF3 (119.7 ppm) or 4-CF3 (121.6 ppm). These results show that subtle differences to the electronics of the system can have major implications on the ligand properties of azophosphines, which is supported by the relatively minor deviations of the 1 JP-Se coupling constants of the phosphine-selenide compounds 3-R. ). In all cases, the monodentate complex 4-R is favoured over the analogous bidentate complex Bid-R (see SI for all values); this is in line with the experimental data, in which the bidentate complexes do not form spontaneously from the corresponding monodentate complex and a halide-abstracting agent is required to promote the reaction. However, the bidentate complexes become less disfavoured when electrondonating substituents are used (Figure ), which is rationalised by the donor sites on the azophosphine becoming more electron-rich. These data highlight the importance of electron-donating parasubstituents when forming complexes with azophosphines. Despite the formation of the bidentate complexes being thermodynamically uphill relative to the monodentate complexes, this reaction is driven by precipitation of the corresponding NaCl salt following halide abstraction, and subsequent formation of an intermediate (Int-R; Figure ). For all the R groups explored, Bid-R is more energetically favoured than the corresponding Int-R; for R = NMe2 this value is 6.26 kcal.mol -1 , whereas for the electron-withdrawing R = CF3 this value is only 3.38 kcal.mol -1 (see SI for all values; the reaction profile for the NMe2-substituted compounds is shown in Figure ). To extend this computational insight, in our previous publication we explored the complexation of the Nmesityl substituted azophosphine, MesN2P t Bu2, and showed that although it was simple to access the monodentate Ru complex Ru(p-cymene)(κ 1 P-(MesN2P t Bu2))Cl2 (c.f. 4-R), despite extensive synthetic efforts we could not synthesise the bidentate analogue (c.f. 5-R). We therefore carried out the same computational study on this particular azophosphine and found that the final bidentate product is actually higher in energy than the halideabstracted intermediate by 6.49 kcal.mol -1 , so although halide abstraction is experimentally possible, the formation of the bidentate complex is disfavoured. This result shows that the steric profile of the N-aryl group on the azophosphine is key, and in the mesityl case the ortho-methyl substituents are bulky enough to preclude formation of the bidentate complex.