The novel, hemilabile (phosphinoalkyl)arene ligands ArX(CH2)2PPh2 (1a, Ar = C6H5, X = O; 1b, Ar = C6H5, X = CH2; 1c, Ar = FC6H4, X = CH2) were synthesized and complexed to Rh(I) to form the bis(phosphine), η6-arene piano stool complexes [(η6:η1-ArX(CH2)2PPh2)Rh(η1-ArX(CH2)2PPh2)]BF4 (2a−c). Complexes 2a−c were fully characterized in solution, and complex 2a was characterized by single-crystal X-ray diffraction methods. Two of these complexes, 2a,c, undergo an unusual, degenerate η6-arene, free arene exchange reaction which was studied by 2-D NMR EXSY experiments. A mechanism for the exchange reaction of 2a which involves the formation of a square planar, cis-phosphine, cis-ether Rh(I) complex, [Rh(η2-PhO(CH2)2PPh2)2]BF4 (13), is proposed.

The synthesis, characterization, and electrochemistry of a series of Rh(I) bis(phosphine), η 6 -arene, pianostool complexes are reported. The study reported herein elucidates several of the important factors which lead to the stabilization of Rh(II) in this coordination environment. From the electrochemical data for a series of complexes of the type [Rh(η 2 -dppe)(η 6 -C 6 H 6-n X n )]BF 4 (X ) CH 3 , n ) 0-6) (1-7) it was shown that the addition of methyl groups to the arene ligand kinetically stabilize the Rh(II) center and thermodynamically stabilize the Rh(II) species by 16 mV per added methyl group. Furthermore, complexes which contain chelation to the arene ligand, such as [Rh(η 6 :η 1 -Ph(CH 2 ) 3 PPh 2 )(η 1 -Ph(CH 2 ) 3 PPh 2 )]BF 4 (12), kinetically stabilize the Rh(II) form, presumably from ligand substitution based decomposition reactions. The electrochemical studies of five isostructural and isoelectronic complexes, [Rh(η 2 -dppe)(η 6 -C 6 H 5 CH 3 )]BF 4 (2), [Rh(η 1 -n-BuPPh 2 ) 2 (η 6 -C 6 H 5 CH 3 )]BF 4 (8), [Rh(η 2 -dppp)(η 6 -C 6 H 5 -CH 3 )]BF 4 ( 9), [Rh(η 2 -dppb)(η 6 -C 6 H 5 CH 3 )]BF 4 (10), and 12 show that those complexes which contain bidentate, bis(phosphine) chelation with an ethyl or butyl bridge, 2 and 10, thermodynamically destabilize the Rh(II) form relative to those complexes which contain a less restricted bis(phosphine) chelate or no bis(phosphine) chelation. Using single-crystal X-ray data and extended Hu ¨ckel calculations, these counterintuitive electrochemical trends were explained in terms of not only the properties of the Rh(I) complex but also, in terms of the structural changes which are likely to occur upon oxidation of the metal center from Rh(I) to Rh(II).

Electrochemical control over the ligand sphere of transition metals is achieved with a new class of redox‐switchable hemilabile ligands (RHL). Electrochemical oxidation of the RHLRh1 complex 1 results in a dimerization to 2. In solution at low concentrations, 1 is the dominant species, while at higher concentrations and in the solid state, the reduced form of 2 (= 2 + 4e−) is favored. Fc = ferrocenyl; the counterion is BF 4−.

Which arene ligand coordinates? An intramolecular exchange previously unknown for RhI complexes in which the phenyl ether moieties bind alternately in an η6 fashion to the Rh1 center (in 1 and 1' on the right) is observed by two‐dimensional 1HNMR exchange spectroscopy. This exchange of the aryl ether‐phosphane ligands probably proceeds via intermediate 2 and therefore cannot be detected when the O atoms are exchanged for CH2 groups.