STRUCTURAL SYSTEMATICS .5. CONFORMATION AND BONDING IN THE CHIRAL METAL-COMPLEXES [M(ETA-5-C5R5)(XO)Z(PPH3)]

Data were retrieved from the Cambridge Structural Database for 67 crystal structures containing suitable geometric data for 74 molecular fragments of the form [M(eta5-C5R5) (XO)Z(PPh3)] 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 PPh3; and group II which has S configuration at the metal and clockwise propeller (right-hand helix) PPh3. Group I structures predominate in the available dataset (56 members to 18 in group II). In group I structures the phenyl group closest to the Z and XO ligands (ring B) of the PPh3 ligand is on average rotated ca. 35-degrees away from Z towards the XO ligand and is face-on to Z. In group II structures phenyl B is edge-on to Z and face-on to XO, on average being rotated a further 36-degrees from Z. Analysis of the conformational data suggests the following order for the energy barrier of rotational processes involving the PPh3 ligand: rotation about the M-P bond > PPh3 helicity inversion by one-ring flip > PPh3 helicity inversion by asynchronous two-ring flip. Helicity inversion can occur without full rotation about the M-P bond and is accompanied by a rocking about the M-P bond. Full rotation about the M-P bond takes place with concomitant PPh3 helicity inversion. This analysis of experimental data provides a detailed test of a previous conformational model. The PPh3 groups show distortions in M-P and P-C distances and C-P-C angles consistent with significant and asymmetric M-P pi bonding in those complexes for which Z is a poor pi acceptor and provide evidence for a model of PPh3 pi back bonding which involves participation of P-C sigma* orbitals.