The effects of variation in the steric and electronic properties of a number of heterocyclic unidentate tertiary phosphine ligands on their ability to form five-co-ordinate tris (phosphine) complexes with nickel (II), palladium (II), and platinum (II) dihalides, have been studied. In contrast to 5-methyl-and 5-ethyl-5H-dibenzophosphole (I; R= Me, Et) which give rise to a series of five-co-ordinate complexes with the above halides, both 5-isopropyl-and 5-t-butyl ...