Data were retrieved from the Cambridge Structural Database for 67 crystal structures containing suitable geometric data for 74 molecular fragments of the form [M(q5-C5R5) (XO)Z(PPh,)] 1 (X = C or N; Z = any ligand). These data were analysed to examine the conformational preferences for the triphenylphosphine ligand in respect of orientations about the M-P bond and about the P-C bonds of the attached phenyl groups, and the mechanisms by which the preferred conformers interconvert. There are two main diastereomeric conformers: group I which has S configuration at the metal and anticlockwise propeller (left-hand helix) conformation at PPh,; and group I I which has S configuration at the metal and clockwise propeller (right-hand helix) PPh,. Group I structures predominate in the available dataset (56 members to 18 in group 11). In group I structures t h e phenyl group closest to the Z and XO ligands (ring B) of t h e PPh, ligand is on average rotated ca. 35" away from Z towards the XO ligand and is face-on to 2. In group II structures phenyl B is edge-on to 2 and face-on to XO, on average being rotated a further 36" from 2.Analysis of the conformational data suggests the following order for the energy barrier of rotational processes involving t h e PPh, ligand: rotation about the M-P bond > PPh, helicity inversion by one-ring flip > PPh, helicity inversion by asynchronous two-ring flip. Helicity inversion can occur without full rotation about t h e M-P bond and is accompanied by a rocking about t h e M-P bond. Full rotation about the M-P bond takes place with concomitant PPh, helicity inversion. This analysis of experimental data provides a detailed test of a previous conformational model. The PPh, groups show distortions in M-P and P-C distances and C-P-C angles consistent with significant and asymmetric M-P K bonding in those complexes for which Z is a poor R acceptor and provide evidence for a model of PPh, R back bonding which involves participation of P-C o* orbitals.

The Ru=Ru double-bonded p-alkynecomplexes [Ru,(p-CO)(p-RC,R)(q-C5H5),] ( R = Ph or CF,) react in heptane or xylene at reflux with alkynes R'CECR' (R' = C0,Me or CF,) to link up to four molecules of alkyne; the structures of the complexes [ R U , ( C O ) { ~-C , ( C F , ) , ) ( ~-C ~H ~) ~] (two isomers) and [Ru,{p-C,(CF,),(CO,Me),}(q-C,H,),],which contains a C, ring with a C, 'tail', have been established by X-ray diffraction.

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The Ru=Ru double-bonded p-alkyne complexes [ R u ~( ~-C O ) ( ~-R C ~R ) ( ~-C ~H ~) ~] react slowly or not at all with ethene and alkynes even under forcing conditions, but in the presence of acid C-H activation of ethene and alkyne-linking occur rapidly at room temperature to give di-p-vinyl complexes [ R u ~( C O ) ( ~-C R = C H R ) ( ~-C H = C H ~) ( ~-C ~H ~) ~] and metallacyclopentadiene complexes [Ru2(CO)( p-C4R4)(q-C5H5)2] respectively; the acid catalysis is shown to proceed via the formation of a Ru-Ru triple-bonded p-vinyl cation [ R ~~( p -c O ) ( p -c R = c H R ) ( q -c ~H ~) ~] + .