Structural systematics. Part 5. Conformation and bonding in the chiral metal complexes [M(η5-C5R5)(XO)Z(PPh3)]

Abstract

Date were retrieved from the Cambridge Structural Database for 67 crystal structures containing suitable geometric data for 74 molecular fragments of the form [M(η⁵-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° 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° 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 π bonding in those complexes for which Z is a poor π acceptor and provide evidence for a model of PPh₃π back bonding which involves participation of P–C σ* orbitals.