Question

I've observed different chemical shifts in 27Al Nuclear Magnetic Resonance (NMR) spectra of commercial aqueous sodium aluminate, indicating the presence of both trivalent [Al(III)] and tetravalent [Al(IV)] aluminium bonds with different numbers of other atoms which is consistent with 27Al NMR seen in literature (Source1, Source2). However, considering aluminium's position in the periodic table with the Al3+

charge (which is not 4+), why isn't Al(III) the only existing valence state for aluminium? What factors lead to the coexistence of both trivalent and tetravalent forms of Al?

Answer 1

The coexistence of trivalent and tetravalent forms of aluminum is due to the inert pair effect, influenced by factors such as the charge and size of the metal ion.

Step-by-step explanation:

The coexistence of trivalent and tetravalent forms of aluminum can be attributed to the phenomenon called the inert pair effect. In this effect, the valence s orbital of the aluminum atom becomes more stable when it loses two electrons instead of three. This leads to the formation of tetravalent aluminum compounds, alongside the more commonly observed trivalent compounds.

Factors that contribute to the coexistence of both forms include the charge on the metal ion and the size of the ion. Divalent ions exert a stronger effect on the electron density in a coordinated water molecule compared to monovalent ions. Additionally, smaller metal ions have a greater effect on the electron density distribution in the water molecule.

In summary, the coexistence of trivalent and tetravalent forms of aluminum is due to the inert pair effect, which causes the aluminum atom to preferentially form tetravalent compounds when possible, despite its usual trivalent oxidation state.