The [N(PPh ₂ Se) ₂ ] ^- ion formed stable bis-chelates with M ^II (M = Sn, Pb, Zn, Cd or Hg), [M{N(PPh ₂ Se) ₂ -Se,Se′ } ₂ ], which exhibit a large diversity of structural arrangements around the metal, as shown by X-ray crystallography. The tin(II) complex crystallises dimorphically, as yellow triclinic prisms (1) and red tetragonal octahedrons (1′). The yellow isomer is square planar, representing the first example of a true square-planar tin(II) spiro-compound. The red isomer and the lead(II) compound (2) exhibit a distorted ψ-trigonal-bipyramidal co-ordination geometry with a vacant equatorial position, which suggests the presence of a stereoactive electron lone pair. The cadmium(II) and mercury(II) complexes (3 and 4) are tetrahedral.

The yellow complex [Sn{N(SePPh₂)₂-Se,Se′}₂] is the first example of a true square-planar spiro tin(II) compound; the structure of the distorted tetragonal-pyramidal isomer is also described.