Chemistry · Chemical Bonding

Nyholm and Gillespie (1957) refined the VSEPR model by explaining the important difference between the lone pair and bonding pairs of electrons.

While the lone pairs are localised on the central atom, each bonded pair is shared between two atoms. As a result, the lone pair electrons in a molecule occupy more space as compared to the bonding pairs of electrons. This results in greater repulsion between lone pairs of electrons as compared to the lone pair - bond j pair and bond pair - bon pair repulsions. These repulsion effects result in deviations- from idealised shapes and alterations in bond angles in molecules.

Nyholm and Gillespie's theory only explains the electronic and magnetic properties of transition metal complexes, but not their molecular shapes.

Nyholm and Gillespie's theory explains the shapes of molecules for non-transition elements based on the repulsion between electron pairs in the valence shell.

Nyholm and Gillespie's theory does not account for the low electron density in the outermost d-orbital of alkali metals, leading to deviations in their predicted molecular shapes.

Nyholm and Gillespie's theory does not explain the shapes of molecules for alkaline earth metals because they do not take into account the participation of d-orbitals in bonding for these elements.