Geometry optimization procedures and electronic structure SCF/MRCl calculations of the intermediate neglect differential overlap (INDO) type are used to study the interaction between a Sn(CH3)(4) molecule and a reduced hydrogenated Rh-13 I-b cluster, whose importance is associated to the formation of an active and selective catalytic phase for hydrogenation processes. According to the calculations, the reaction mechanism implies the initial adsorption of alkyl tin to a Rh atom of the hydrogenated metal cluster, followed by the formation of several intermediate structures through the successive cleavage of Sn-C bonds. In agreement with experimental data we found an oxidation state zero for the adsorbed Sn atom in the final structure, which is compatible, according to the adsorption geometry, with the coverage of 0.66 (Sn/Rh).