Complementary spherical electron density model and its implications for the 18 electron rule

The Complementary Spherical Electron Density Model developed by Mingos and Hawes in 1985 is reviewed and its implications to 18 electron complexes and co-ordinatively unsaturated complexes are developed. These conclusions are particularly germane in view of the recent proposal by Landis that transition metal complexes are governed by a 12 rather than an 18 electron rule, Hall’s recent OSRAM model and the availability of additional structural data.