The conformational analysis of phosphine ligands in organometallic complexes. Part 2. Triphenylphosphine coordinated to achiral and prochiral octahedral metal centres
Costello JF., Davies SG., McNally D.
The novel concept of nadir energy planes has been used to illuminate the principles governing the preferred orientation of rings A-C in PPh3 1, coordinated to achiral octahedral metal complexes of the type [M(PPh3)L1-5] 2a-d, and prochiral octahedral metal complexes of the type [M(η5-C5H5)(PPh3)L2] 3, and [M(η6-C6H6)(PPh3)L2] 4. An achiral arrangement of complex-bound 1 (reminiscent of the transition state for the one-ring flip stereoisomerisation process) orients all three ring apices proximal to three of the four orthogonal nadir energy planes associated with an octahedral metal centre. From this achiral arrangement, the favoured, degenerate conformations of complex-bound 1 may be subsequently derived by applying the following principles (in descending order of priority), (i) superimpose the vertical ring A of the achiral conformer onto the least encumbered nadir plane, (ii) allow ring B to tilt onto the least encumbered nadir plane orthogonal to the vertical ring A, and (iii) tilt ring B in a manner which orients the flatter ring C beneath the smallest ligand. These principles are wholly consistent with 60 X-ray crystal structures, and detailed conformational analyses. The phenomenon of PPh3 ligand tilting, consistent with intramolecular steric strain, is characterised.