Intramolecular Alkyl Phosphine Dehydrogenation in Cationic Rhodium Complexes of Tris(cyclopentylphosphine)

[Rh(nbd)(PCyp 3 ) 2 ][BAr F 4 ] ( 1 ) [nbd = norbornadiene, Ar F = C 6 H 3 (CF 3 ) 2 , PCyp 3 = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCyp 3 ligand in CH 2 Cl 2 solution to form an equilibrium mixture of cis ‐[Rh{PCyp 2 (η 2 ‐C 5 H 7 )} 2 ][BAr F 4 ] ( 2 a ) and trans ‐[Rh{PCyp 2 (η 2 ‐C 5 H 7 )} 2 ][BAr F 4 ] ( 2 b ), which have hybrid phosphine–alkene ligands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2 b is distorted in the solid‐state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H 2 to 2 a / b hydrogenates the phosphine–alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCyp 3 ) 2 (H) 2 (η 2 ‐H 2 ) 2 ][BAr F 4 ] ( 5 ) which has been identified spectroscopically. Addition of the hydrogen acceptor tert ‐butylethene (tbe) to 5 eventually regenerates 2 a / 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 (η 2 ‐C 5 H 7 )}(PCyp 3 )(NCMe)][BAr F 4 ] ( 6 ). Dehydrogenation of a PCyp 3 ligand also occurs on addition of Na[BAr F 4 ] to [RhCl(nbd)(PCyp 3 )] in presence of arene (benzene, fluorobenzene) to give [Rh(η 6 ‐C 6 H 5 X){PCyp 2 (η 2 ‐C 5 H 7 )}][BAr F 4 ] ( 7 : X = F, 8 : X = H). The related complex [Rh(nbd){PCyp 2 (η 2 ‐C 5 H 7 )}][BAr F 4 ] 9 is also reported. Rapid (≈5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp 3 )] with Na[BAr F 4 ] to give [Rh(dppe){PCyp 2 (η 2 ‐C 5 H 7 )}][BAr F 4 ] ( 10 ), which reacts with H 2 to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp 3 )(H) 2 (η 2 ‐H 2 )][BAr F 4 ] ( 11 ). Competition experiments using the new mixed alkyl phosphine ligand PCy 2 (Cyp) show that [RhCl(nbd){PCy 2 (Cyp)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give [Rh(η 6 ‐C 6 H 5 X){PCy 2 (η 2 ‐C 5 H 7 )}][BAr F 4 ] ( 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 2 a / b , 4 , 6 , 7 , 8 , 9 and 17 also by single crystal X‐ray diffraction.