The reaction of 2 with half an equivalent of the [Ru(pcymene)Cl2]2 dimer (Scheme 3) was monitored by 31 P{ 1 H} NMR spectroscopy, and clean conversion of the free azophosphine at 118.3 ppm to a new species at 121.7 ppm was observed. SXRD showed that this product was the half-sandwich piano stool complex Ru(p-cymene)(κ 1 P-2)Cl2 (4), which featured the azophosphine coordinated to the metal centre exclusively via the phosphine centre (Figure ). The only prior example of a coordinated azophosphine was the unexpected reaction of a PF3 ligand in [RhCl(PF3)(PPh3)2] with a silyldiazene, which generated the P,P-difluoroazophosphine PhN=NPF2 bound to Rh; this product was only characterised by NMR spectroscopy and elemental analysis, but the results were consistent with Pcoordination of the ligand. Scheme 3. Synthesis of Ru(arene) azophosphine complexes;