Tl(acac) reacts quantitatively with B₂butcat₃ (butcat = 3,5-di-tert-butylcatecholato) to give butcatB(acac) and Tl(Bbutcat ₂) (1), the latter of which is soluble in common organic solvents such as THF and dichloromethane. Addition of 1 to a mixture of [RhCl(coe) ₂]₂ (coe = cis-cyclooctene) and diphosphine in THF gave the thallium bridged dinuclear complexes [(P₂)Rh(μ-Cl) ₂(μ-Tl)Rh(P₂)][Bbutcat₂] (2-5, P₂ is dppm = 1,1′-bis(diphenylphosphino)methane, dppe = 1,2- bis(diphenylphosphino)ethane, dppp = 1,3-bis(diphenylphosphino)propane and dppb = 1,4-bis(diphenylphosphino)butane) as the only new rhodium containing species. Unsymmetrical binding of the thallium atom is observed in some cases owing to a secondary interaction with the phenyl rings of the phosphine ligands. Mononuclear cationic species of the type [(P₂)Rh(NCCH ₃)₂][Bbutcat₂] (6-9) were formed when reactions were carried out in acetonitrile, where preferential binding of the solvent was observed over the bulky arylspiroborate ligand. The zwitterionic species Rh(η⁶-butcatBbutcat)(P₂), 11 and 12, could be generated via a different route, by addition of B₂butcat₃ to Rh(acac)(P₂), but only for the sterically-constrained diphosphines dppm and dppe, respectively. While all new complexes effectively catalyzed the addition of catecholborane to 4-vinylanisole and α-methylstyrene, product distributions suffered from a competing dehydrogenative borylation pathway.