A rhodium(H) dibenzotetramethylaza[14]-annulene dimer ([(tmtaa)Rh](2)) (1) reacts with CO and H-2 in toluene and pyridine to form equilibrium distributions with hydride and formyl complexes ((tmtaa)Rh-H (2); (tmtaa)Rh-C(O)H (3)). The rhodium formyl complex ((tmtaa)Rh-C(O)H) was isolated under a CO/H-2 atmosphere, and the molecular structure was determined by X-ray diffraction. Equilibrium constants were evaluated for reactions of (tmtaa)Rh-H with CO to produce formyl complexes in toluene (K-2(298 K)(tol) = 10.8 (1.0) X 10(3)) and pyridine (K-2(298 K)(py) = 2.2 (0.2) x 10(3)). Reactions of 1 and 2 in toluene and pyridine are discussed in the context of alternative radical and ionic pathways. The five-coordinate 18-electron Rh(I) complex ([(py)-(tmtaa)Rh-I](-)) is proposed to function as a nucleophile toward CO to give a two-electron activated bent Rh-CO unit. Results from DFT calculations on the (tmtaa)Rh system correlate well with experimental observations. Reactions of 1 with CO and H-2 suggest metal catalyst design features to reduce the activation barriers for homogeneous CO hydrogenation.