Intramolecular alkyl phosphine dehydrogenation in cationic rhodium complexes of tris(cyclopentylphosphine)

[Rh(nbd)(PCYp(3))(2)][BAr4F] (1) [nbd = norbornadiene, Ar-F = C6H3(CF3)(2), PCYP3 = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCYP3 ligand in CH2Cl2 solution to form an equilibrium mixture of cis-[Rh{PCYp(2)(eta(2)-C5H7)}(2)][BAr4F] (2a) and trans-[Rh{PCYp(2)(eta(2)-C5H7)}(2)][BAr4F](2b), which have hybrid phosphine-alkene ligands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2b is distorted in the solid-state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H-2 to 2a/b hydrogenates the phosphine-alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCYp(3))(2)(H)(2)(eta(2)-H-2)(2)] [BAr4F] (5) which has been identified spectroscopically. Addition of the hydrogen acceptor tert-butylethene (tbe) to 5 eventually regenerates 2a/b, passing through an intermediate which has undergone dehydrogenation of only one PCYp(3) ligand, which can be trapped by addition of MeCN to form trans-[Rh{PCYp(2)(eta(2)-C5H7)}(PCYp(3))(NCMe)][BAr4F] (6). Dehydrogenation of a PCYp(3) ligand also occurs on addition of Na[BAr4F] to [RhCl(nbd)(PCyp(3))] in presence of arene (benzene, fluorobenzene) to give [Rh(eta(6)-C6H5X){PCYp(2)(eta(2)-C5H7)}][BAr4F] (7: X = F, 8: X = H). The related complex [Rh(nbd){PCYp(2)(eta(2)-C5H7)}][BAr4F] 9 is also reported. Rapid (approximate to 5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp(3))] with Na[BAr4F] to give [Rh(dppe){PCyp(2)(eta(2)-C5H7)}][BAr4F] (10), which reacts with H, to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp(3))(H)(2)(eta(2)-H-2)][BAr4F] (11). Competition experiments using the new mixed alkyl phosphine ligand PCy2(CYP) show that [RhCl(nbd){PCy2(CyP)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give [Rh(eta(6)-C6H5X){PCy2(eta(2)-C5H7)}][BAr4F] (17: X = F, 18: X = H). The underlying reasons behind this preference have been probed using DFT calculations. All the complexes have been characterised by multinuclear NMR spectroscopy, and for 2a/b, 4, 6, 7, 8, 9 and 17 also by single crystal X-ray diffraction.