The homologous series of diphosphines (CH2)(n-1)P(CH2)(3)P(CH2)(n-1) where n = 5(L-5), 6(L-6), or 7(L-7) have been synthesized from the corresponding PhP(CH2)(n-1). Treatment of [PtCl2(cod)] with L5-7 gave the 6-membered chelates cis-[PtCl2(L5-7)], the crystal structures for which reveal that L5-7 have very similar steric bulk and bite angles. Treatment of [Rh2Cl2(CO)(4)] with L5-7 gave the binuclear trans-[Rh2Cl2(CO)(2)(m-L5-7)(2)] with syn and anti orientations of the CO and Cl ligands suggested by the P-31 NMR spectra and the crystal structures of syn-trans-[Rh2Cl2(CO)(2)(m-L-5)(2)] and anti-trans[Rh2Cl2(CO)(2)( m-L-7)(2)]. The n(CO) values for trans-[Rh2Cl2(CO)(2)(m-L5-7)(2)] indicate that the donor strength increases in the order L-5 < L-6 < L-7. A study of rhodium-catalysed hydroformylation of 1-octene using diphosphines (L5-7) is described. The catalyst activity decreases with increasing phosphacycle ring size: L-5 > L-6 > L-7.