Phosphine-boranes as bidentate ligands:: Formation of [8,8-η2-η2-(BH3)•dppm}-nido-8,7-RhSB9H10] and [9,9-η2-η2-(BH3)•dppm}-nido-9,7,8-RhC2B8H11] from [8,8-(η2-dppm)-8-(η1-dppm)-nido-8,7-RhSB9H10] and [9,9-η2-dppm)-9-(η1-dppm)-nido-9,7,8-RhC2B8H11], respectively

The two clusters [8,8-(eta(2)-dppm)-8-(eta(1)-dppm)-nido-8,7-RhSB9H10] (1) and [9,9-(eta(2)-dppm)-9-(eta(1)-dppm)-nido-9,7,8-RhC2B8H11] (2) (dppm = PPh2CH2PPh2), both of which contain pendant PPh2 groups, react with BH3.thf to afford the species [8,8-{eta(2)-(BH3).dppm}-nido-8,7-RhSB9H10] (3) and [9,9-eta(2)-{eta(2)-(BH3).dppm}-nido-9,7,8-RhC2B8H11] (4), respectively. These two species are very similar in that they both contain the bidentate ligand [(BH3)-dppm], which coordinates to the Rh center via a PPh2 group and also via a eta(2)-BH3 group. Thus, the B atom in the BH3 group is four-coordinate, bonded to Rh by two bridging hydrogen atoms, to a terminal H atom, and to a PPh2 group. At room temperature, the BH3 group is fluxional; the two bridging H atoms and the terminal H atom are equivalent on the NMR time scale. The motion is arrested at low temperature with DeltaG(double dagger) = ca. 37 and 42 kJ mol(-1), respectively, for 3 and 4. Both species are characterized completely by NMR and mass spectral measurements as well as by elemental analysis and single-crystal structure determinations.