SUMMARY OF KEY CONCEPTS

 

•	Four coordinate metals (ML4) predominantly adopt one of two geometries: square planar or tetrahedral.

•	In some complexes the energy difference between these two preferred geometries is small. Indeed, in a few specific complexes the difference in stability of the two possible stereochemical forms (square planar or tetrahedral) is so small that each can exist in the same crystal structure.

•	Dibromo-bis(benzyldiphenylphosphine) nickel(ii) (CSD refcode DBBZPN) is one such structure. The term allogons (Greek: allos, other, different; andgonia, angle) is used to describe such isomers. This crystal is thus an interallogon compound.

 

Both square planar and tetrahedral forms exist together in dibromo-bis(benzyldiphenylphosphine) nickel(ii) (CSD refcode DBBZPN)

 

•	The preferred square planar and tetrahedral geometries can be affected by e.g. the nature of the substituents, and thus not all metal complexes have idealised conformations. In fact there are many database entries linking the two idealised geometries resulting in a continuum.

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The structures which are neither square planar nor tetrahedral can be regarded as snapshots of transition states along the interconversion pathway. i.e. proceeding along the continuum in the Vista distribution and viewing successive entries will allow the pathway for the interconversion between the square planar and tetrahedral geometries to be mapped out.