The facile synthesis of the metallaheteroborane [8,8-(PPh 3) 2- nido-8,7-RhSB 9H 10] ( 1) makes possible the systematic study of its reactivity. Addition of pyridine to 1 gives in high yield the 11-vertex nido-hydridorhodathiaborane [8,8,8-(PPh 3) 2H-9-(NC 5H 5)- nido-8,7-RhSB 9H 9] ( 2). 2 reacts with C 2H 4 or CO to form [1,1-(PPh 3)(L)-3-(NC 5H 5)- closo-RhSB 9H 8] [L = C 2H 4 ( 3), CO ( 4)]. In CH 2Cl 2 at reflux temperature 2 undergoes a nido to closo transformation to afford [1,1-(PPh 3) 2-3-(NC 5H 5)- closo-1,2-RhSB 9H 8] ( 5). Reaction of 2 with alkenes leads to hydrogenation and isomerization of the olefins. NMR spectroscopy indicates the presence of a labile phosphine ligand in 2, and DFT calculations have been used to determine which of the two phosphine groups is labile. Rationalization of the hydrogenation mechanism and the part played by the 2 --> 3 nido to closo cluster change during the reaction cycle is suggested. In the proposed mechanism the classical hydrogen transfer from hydride metal complexes to olefins occurs twice: first upon coordination of the alkene to the rhodium centre in 2, and second concomitant with formation of a closo-hydridorhodathiaborane intermediate by migration of a BHB-bridging hydrogen atom to the metal. Reaction of H 2 with 3 or 5 regenerates 2, closing a reaction cycle that under catalytic conditions is capable of hydrogenating alkenes. Single-site versus cluster-bifunctional mechanisms are discussed as possible routes for H 2 activation.