The syntheses, spectral properties, and molecular structures of compounds isolated from the reaction of Rh-2(O2CCF3)(4)(acetone)(2) with the ether phosphine tris (2,4,6-trimethoxyphenyl) phosphine (TMPP) are described. Two products are formed, the relative yields of which are determined by reaction conditions and stoichiometry. The tetranuclear compound [{Rh-2(O2CCF3)(3)[{C6H2(OMe)(3)}P-2(C6H2(OMe)(2)O}](2) (1) is favored under mild conditions with 2 equiv of phosphine ligand whereas the dinuclear product Rh-2(O2CCF3)(2){C6H2(OMe)(3)}P-2(C6H2(OMe)(2)O}(2) (2) is obtained from further reaction of 1 with TMPP or by reaction of the trifluoroacetate complex with 4 equiv of TMPP. The excess phosphine in these reactions aids in the demethylation of an ortho-methoxy substituent and, if absent, results in a low yield of substitution products. Compound 1 crystallizes as a CH2Cl2 solvate in the monoclinic space group P2(1)/n with unit cell dimensions a = 14.718(7) Angstrom, b = 24.040(5) Angstrom, c = 24.260(6) Angstrom, beta = 98.78(3)degrees, V = 8484(5) Angstrom 3, and Z = 4. Final refinement of 937 parameters gave residuals of R = 0.058 and R(w) = 0.090 with a quality-of-fit = 3.06. An X-ray structure revealed that the molecule exhibits a new binding mode for the TMPP ligand wherein a phenoxide bridge spans two dirhodium units to give a "dimer-of-dimers". The compound dissolves in coordinating solvents with disruption of the intermolecular Rh-phenoxide bonds to give two molecules of the dinuclear species Rh-2(O2CCF3)(3)(TMPP-O)(S) (S = solvent).The tetranuclear structure of 1 is in equilibrium with the dimer form in noncoordinating solvents such as CHCl3, as evidenced by two independent sets of phosphine resonances in the H-1 NMR and P-31 NMR spectra. Compound 2 crystallizes as a CH2Cl2 solvate in. the monoclinic space group P2(1)/n with unit cell dimensions a = 14.146(4) Angstrom, b = 19.56(1) Angstrom, c = 23.822(6) Angstrom, beta = 101.90(2)degrees, V = 6449(4) Angstrom 3, and Z = 4. Residuals of R = 0.066 and R(w) = 0.099 were obtained with 805 parameters and a quality-of-fit = 2.94. In 2, each Rh center is bonded to a face-capping TMPP (eta 3-P,O,O) and to cis-trifluoroacetate ligands in the usual bridging mode. The solid-state structure of 2 is preserved in solution, as judged by NMR spectroscopy, which reveals one type of TMPP ligand with magnetically inequivalent arene The: results of these studies are in contrast to previous findings for the the corresponding reactions of Rh-2(O2CCH3)(4) with TMPP that produce a dinuclear complex with the ligand arrangement found in 1 but no tetranuclear or bis-substituted products. Details of the syntheses, full characterization of compounds 1 and 2, and a comparison of this work to related chemistry are described. groups.