Transition‐metal complexes with bidentate ligands spanning trans‐positions. Part XVIII. Crystal and molecular structure of the ligands 2,11‐bis(di‐R‐phosphinomethyl)benzo[c]phenanthrene (R = Phenyl, tert‐Butyl) and a comparison of their conformation in a variety of complexes

The X‐ray crystal structures of 2,11‐bis(di‐R‐phosphinomethyl)benzo[ c ]phenanthrenes (R = Ph, t ‐Bu) are reported. Both ligands have the same conformation and the substitution of a Ph by a t ‐Bu group does not cause significant changes in the tetracyclic aromatic unit which shows the same distortions as the parent hydrocarbon. The conformations and structural features of the two ligands and 18 complexes containing these ligands as well as of one compound with a ligand with Et substituents are compared. It is pointed out that ( 1 ) the conformations of the free ligands differ significantly from those of their complexes, ( 2 ) in all the complexes characterized to date, these ligands act as chelating agents, and ( 3 ) the ligands adopt two different conformations in the complexes, one of them appearing to be more flexible than the other.