The syntheses, spectral properties, redox chemistry, and structures of complexes obtained from the reaction of carbon monoxide with a rare mononuclear Rh(II) species are described. The ionic compound [Rh(eta-3-TMPP)2][BF4]2 (1) {TMPP = tris(2,4,6-trimethoxyphenyl)phosphine, P(C6H2(OMe)3)3} has been prepared in high yield by reaction of the unbridged Rh2(4+) complex [Rh2(CH3CN)10][BF4]4 with 4 equiv of TMPP in CH3CN. The product crystallizes in the orthorhombic space group Pbcn with unit cell dimensions a = 15.938 (5) angstrom, b = 17.916 (7) angstrom, c = 21.015 (8) angstrom, V = 5376 (3) angstrom 3, and Z = 4. Residuals of R = 0.073 and R(w) = 0.089 were obtained after least-squares refinement of 369 parameters to convergence and a quality-of-fit of 2.37. The Rh atom, which is situated on a 2-fold axis, is bonded to two ether phosphine groups in a capping, tridentate mode through the phosphorus atom and two o-methoxy substituents. The geometry about the metal is pseudooctahedral, and, surprisingly, the phosphorus atoms lie cis to one another. A Jahn-Teller structural distortion was observed for 1 along the direction of the two trans oxygen atoms, which constitutes the first documentation of this effect in a molecular Rh(II) d7 complex. [Rh(eta-3-TMPP)2][BF4]2 was also characterized by infrared, electronic and EPR spectroscopies, and by elemental analysis. Complex 1 reacts reversibly with carbon monoxide by a series of redox reactions, initiated by a highly unstable Rh(II) dicarbonyl adduct. Products isolated from the reaction under a CO atmosphere are the Rh(III) complex [Rh(eta-3-TMPP)2][BF4]3 (2) and the Rh(I) dicarbonyl complex [Rh(TMPP)2(CO)2[[BF4] (3). Compound 3 has been crystallized as its CH2Cl2 solvate in the space group P1BAR with unit cell dimensions a = 13.318 (4) angstrom, b = 13.404 (2) angstrom, c = 18.104 (4) angstrom, alpha = 95.908 (3)-degrees, beta = 97.037 (3)-degrees, gamma = 90.711 (3)-degrees, V = 3200 (2) angstrom 3, and Z = 2. Least-squares refinement of 779 parameters gave residuals of R = 0.059 and R(w) = 0.084 and a quality-of-fit = 2.316. The Rh(I) cation occupies a general position and exhibits a trans square-planar coordination geometry, with the phosphine ligands acting as monodentate groups. In the absence of a CO atmosphere, 3 readily loses a molecule of CO to form the monocarbonyl species [Rh(eta-2-TMPP)(TMPP)CO][BF4] (4) which has been structurally characterized as the benzene solvate [Rh(eta-2-TMPP)(TMPP)CO][BF4].2C6H6; triclinic space group P1BAR with unit cell dimensions of a = 14.898 (5) angstrom, b = 18.060 (8) angstrom, c = 14.343 (4) angstrom, alpha = 96.56 (4)-degrees, beta = 113.84 (2)-degrees, gamma = 104.80 (4)-degrees, V = 3308 (2) angstrom 3, and Z = 2. Final refinement of 757 parameters gave residuals of R = 0.067 and R(w) = 0.069 with a quality-of-fit of 2.59. The immediate coordination sphere of the Rh atom consists of one CO ligand and two trans phosphorus atoms with the fourth site being occupied by a pendant methoxy group. The resulting structure is a highly distorted square-planar arrangement as evidenced by the bond angle O(1)-Rh(1)-C(55) of 150.2 (4)-degrees. Complex 4 reversibly adds CO, both in the solid state and in solution, to reform 3. Furthermore, [Rh(eta-2-TMPP)(TMPP)CO]1+ (4) undergoes a second electron-transfer process with [Rh(eta-3-TMPP)2][BF4]3 (2) to form [Rh(eta-3-TMPP)2]2+ (1) and the unstable Rh(II) complex [Rh(TMPP)2(CO)]2+. The latter can easily lose CO to regenerate the original Rh(II) radical species 1 or add a second CO, thereby continuing the cycle. The quantitative regeneration of the parent Rh(II) complex is ultimately limited by the thermal instability of [Rh(eta-3-TMPP)2]3+, which undergoes a demethylation reaction at room temperature to give the Rh(III) species [Rh(eta-3-TMPP)(P{C6H2(OMe)2O}{C6H2(OMe)3}2)][BF4]2 (5) with a phenoxide interaction. Compound 5 has been fully characterized by NMR, electrochemistry, and elemental analysis. Complexes 2-5 have also been prepared in high yield by separate routes and independently characterized.
