Some members of the family of coordinatively unsaturated rhodium hydride dimers of the formula [{R 2P(CH 2) n PR 2}Rh] 2(μ-H) 2 ( 1a: n = 2, R = Pr i ; 1b: n = 3, R = Pr i ) were found to react stoichiometrically with olefins and dienes. The reactivity towards olefins is influenced by the steric properties of the ancillary bisphosphine ligand. Compound 1a reacts with a variety of simple olefins to yield binuclear bridging alkenyl-hydride complexes of the formula [(dippe)Rh] 2(μ-η 2-σ-RCCH(R)(μ-H), whereas 1b, which incorporates the more sterically bulky dippp ligand, reacts only with ethylene. The μ-alkenyl ligand in these complexes exhibits the well known “windshield wiper” fluxionality, with a Δ G‡ of 12.2(2) kcal mol −1 found for [(dippe)Rh] 2(μ-η 2-σ-HCCH 2)(μ-H), 2a. The proposed mechanism for these reactions involves a dehydrogenation of the dihydride by one equivalent of olefin, followed by reaction of a second equivalent of alkene with the transient [(P 2)Rh] 2 d 9- d 9 Rh(O) dimer. Dihydrides 1a and 1b also react with 1,3-dienes (butadiene, isoprene and piperylene) to afford binuclear products. Two distinct bonding modes for the diene ligand are found: a μ-η 4-σ mode, and a μ-η 3-η 3 mode in which the diene ligand is partially sandwiched between the two metal centres. The distribution of products is governed by the steric requirements of both the chelating phosphine ligand and the diene. In some cases, mononuclear methyl substituted allyl complexes are produced either as side products, or as the major products. The binuclear compounds may alternatively be synthesized via reaction between the chlorobridged dimers [{R 2P(CH 2) n PR 2Rh] 2(μ-C1) 2 ( n = 2,3; R = Pr z i ) and magnesium diene reagents; indeed this route provides an avenue to compounds not accessible via the dihydride/diene reaction.