Structural systematics. Part 3. Geometry deformations in triphenylphosphine fragments: a test of bonding theories in phosphine complexes
Data were retrieved from the Cambridge Structural Database for 1292 crystal structures containing suitable geometric data for 1860 unique ZPPh3 fragments and analysed to examine the symmetrical (C3v) distortions of the ZPC3 moieties. Strong negative correlation between the mean P–C distance, d, and the mean C–P–C angle, β is observed. The mean values of these parameters for a given Z depend on Z. Strongly σ-bonding substrates Z [such as H+, R+(R = alkyl), O, etc.] are associated with small d(1.79–1.80 Å) and large β(107–110°). For Z = transition metal, larger d(ca. 1.83 Å) and smaller β(102–104°) are observed, leading to geometries close to that of free PPh3. Complexes of the main-group metals and the late- and post-transition metals show PPh3 geometries which are intermediate between these extremes. Monotonic changes in mean values of d and β are observed across the series Cr to Zn, Ru to Cd, and Re to Hg. A qualitative molecular orbital theory that is consistent with these observations is outlined. According to this model, σ donation from phosphorus to Z results in a PPh3 geometry with small d and large β, while π donation from Z to phosphorus yields a geometry with large d and small β. For Z = an element in the first two short periods, σ effects on ZPPh3 geometry predominate, while for Z = transition metal, σ and π effects are broadly in balance. These effects are transmitted to the phenyl rings for which the average C–C–C angle at the ipso carbon is correlated to the P–C distance and C–P–C angle. The structures of other phosphines, PA3, in their complexes, ZPA3, were assessed in a similar way. These also show a pronounced negative correlation between d and β. The tabulated cone angles for these phosphines are shown to be highly correlated to the difference between the sums of Z–P–A and A–P–A angles, suggesting both the importance of steric effects in determining phosphine geometry and and alternative approach to the estimation of ligand bulkiness.