Data were retrieved from the Cambridge Structural Database for 405 crystal structures containing suitable geometric data for either M₂(µ-dppm)1(dppm = Ph₂PCH₂PPh₂) fragments (409 located) or M(dppe)2(M = transitional metal, dppe = Ph₂PCH₂CH₂PPh₂) fragments (274 such located). These data were analysed to examine the conformational preferences for the five-membered MP₂C₂(or M₂P₂C) rings and the attached phenyl groups. The technique of principal component analysis was used to reduce the dimensionality of the torsion angle data set and to aid in identification of favoured conformations. The M(dppe) fragments show a preference for a twist (C₂) conformation of the MP₂C₂ five-membered ring, in which the P–C–C–P torsion angle is far from zero (typically ca. ±50°). In contrast the M₂P₂C five-membered rings of the M₂(µ-dppm) fragments are predominantly of the envelope (C_s) conformation, with the methylene carbon out of the plane of the near-planar M₂P₂ unit, i.e. having the P–M–M–P torsion angle close to zero and the M–P–C–P torsion angles ca. ±45°. The distribution of structures indicates that in both cases the ring conformations interconvert by a pseudo-rotation pathway, similar to those observed for other five-membered ring systems, in which planar intermediates are avoided. In both 1 and 2 the two phenyl groups on each phosphorus adopt conformations typical of one- and two-ring flip mechanisms of rotation. For M₂(µ-dppm) there is considerable constraint of phenyl group orientation due to clashes between phenyl groups in axial sites on the envelope M₂P₂C ring. In contrast, although weak preferences may be seen in the phenyl group conformations for the M(dppe) fragments, these arise primarily because of contacts between the phenyl groups and the ethylene hydrogens of the MP₂C₂ ring and not due to transannular Ph ⋯ Ph contacts. The implications of these observations for the design of new diphosphine ligands are discussed.