On the Instability of Iodides of Heavy Main Group Atoms in their Higher Oxidation State

Inert pair effect—the tendency of s orbital of heavy atoms to stay unreactive, is a consequence of relativistic contraction of the s orbitals. While the manifestations of this, on the reactivity depends on the nature of the substituents, this aspect is often overlooked. Divalent Pb prefers inorganic substituents, whereas tetravalent Pb prefers organic substituents. Among the inorganic substituents, again there are specific preferences—tetravalent Pb prefers F and Cl more than Br and I. It is as though the relativistic contraction of s orbital of Pb is more significant with Br and I substituents, than with Cl, F and alkyl substituents. Herein, we address the problem using molecular orbital approach. We explain why typical hypervalent systems, like 12-X-6, 10-X-5 (X is a heavy atom, the number preceding X is the number of valence electrons surrounding X, and the number after X is the coordination number) with less electronegative substituents carrying lone pair (such as Iodine), and Lewis octet molecules like PbI4 are unstable, but their dianions (14-X-6, 12-X-5, PbI42-) are not. For heavy atoms, the relativistic contraction of s orbital renders the antibonding combination of s with ligand orbitals (σ1*) very low-lying, making it a good acceptor of electrons. Thus compounds where σ1* is empty are kinetically unstable, when an electron donor with appropriate energy (such as lone pair on Iodine or bromine) is present in the vicinity. Donor-acceptor interaction between σ1* and the lone pair on I or Br (F and Cl lone pairs are energetically far away from σ1*) is responsible for the instability of such compounds. The kinetic stability of tetraalkyl lead compounds is due to the absence of lone pairs on the alkyl substituents. This work illustrates the key factor responsible for the instability of heavy element iodides and provides a molecular-level insight on extended solid-state structures via quasi-relativistic density functional computations.