Addition of catecholborane to the low-valent CpRu(PPh(3))(2)Me complex led to formation of the corresponding ruthenium hydride and methylcatecholborane by a mechanism more characteristic of high-valent metal centers. Kinetic experiments conclusively showed that oxidative addition of the B-H bond did not occur. A stereochemical analysis of the reaction employing, (R,R)- and (R,S)-CpRu(DPPP)Me (DPPP = 1,2-bis(diphenylphosphino)propane) showed that epimerization of the ruthenium center was occurring before product formation and that the species undergoing epimerization never reformed starting material. The exchange reaction, therefore, involved an intermediate whose formation was rate determining. The observation of a primary deuterium isotope effect (1.62 +/- 0.13) and the absence of exchange between CpRu(PPh(3))Me-d(3) and MeBcat provided strong evidence for weakening of the borane B-H bond during formation of the transition state leading to this intermediate. The electronic effect of varying the phosphine ligand suggested a buildup of positive charge at the metal center, consistent with an intermediate possessing a coordinated hydridoborate ligand. All of the data were consistent with a mechanism proceeding through a four8centered transition state stat involves partial cleavage of the B-H bond during formation of the B-C bond. Epimerization occurred by dissociation of the resulting hydridoborate ligand to form a three-coordinate ruthenium cation. The intermediate resulting from this step was trapped as a THF complex of the cationic ruthenium center during reactions in this solvent. The unligated three-coordinate cation provided epimerized product, but was not on the pathway to exchange of the boron8hydrogen and ruthenium-methyl groups.