Rh(I) and Ir(I) complexes of the type [Rh(cod)(η 2 -TMPP)] 1 + (1) and M(cod)(η 2 -TMPP-O) (M = Rh (2), Ir (3); cod = cyclooctadiene; TMPP = tris(2,4,6-trimethoxyphenyl)phosphine; TMPP-O = mono-demethylated form of TMPP) have been isolated from reactions of [M(cod)Cl] 2 with M BF 4 (M = Ag + , K + , Na + ) followed by addition of the tertiary phosphine ligand. This chemistry is dependent on the identity of the metal, as both the cationic phosphine complex and the neutral phosphino-phenoxide compound are stable for Rh(I), whereas only the latter is stable for Ir(I). The three complexes have been characterized by IR and NMR ( 1 H and 3 1 P) spectroscopies as well as by cyclic voltammetry. The 1 H NMR spectrum of [Rh(cod) (η 2 -TMPP)] 1 + (1) is in accord with the formula and reveals that the TMPP phenyl rings are undergoing rapid exchange between coordinated and non-coordinated modes; the corresponding spectra of 2 and 3 support free rotation about the P-C bonds of the unbound phenyl rings with no fluxionality of the bound demethylated ring. The 3 1 P{ 1 H} NMR spectrum of the neutral species 2 exhibits a significant upfield shift with respect to the analogous cationic compound 1. This shielding is the result of improved electron donation to the metal from a phenoxide group as compared to an ether substituent. In situ addition of CO to the reaction between TMPP and [Rh(cod)Cl] 2 or [Ir(cod)Cl] 2 in the presence of M BF 4 results in the isolation of the monocarbonyl species [Rh(TMPP) (η 2 -TMPP) (CO)] [BF 4 ] (5) and the stable dicarbonyl compound [Ir(TMPP) 2 (CO) 2 ] [BF 4 ] (4), respectively. Single crystal X-ray data for 4 CH 2 Cl 2 :P1, a = 13.512(2), b = 18.348(3), c = 13.358(2) , α = 97.26(1), β = 90.55(1), γ = 95.02(1)°, V = 3272(2) 3 ,Z = 2, R = 0.051, R w = 0.065. The geometry of 4 is square planar, with essentially ideal angles for the mutually trans disposed phosphine and carbonyl ligands, as found in earlier studies for the analogous Rh dicarbonyl compound. The 1 H NMR spectrum of 4 supports the assignment of magnetically equivalent phosphorus nuclei in solution. The results of this study indicate that cyclooctadiene is a particularly strong ligand for monovalent late transition metals ligated by TMPP, to the extent that it is inert with respect to substitution in the absence of π-acceptor ligands such as carbon monoxide.