Stereochemistry of selenium and tellurium

Selenium and tellurium exhibit a great variety of molecular geometries as a consequence of the number of stable oxidation states. Various observed structures are summarized in Table 16.8.1, in which A is a central Se or Te atom, X is an atom bonded to A, and E represents a lone pair. 

With reference to Table 16.8.1, the stereochemistries of Se and Te compounds are briefly described below. 

Many AX2E2 type molecules, such as Se(CH3)2, SeCl2, Te(CH3)2, and TeBr2, all exhibit a bent configuration, in which repulsion of lone pairs makes the interbond angles smaller than the ideal tetrahedral angle  

Monomeric selenium trioxide (Se03) and tellurium trioxide (Te03) have a trigonal planar structure in the gas phase. In the solid state, Se03 forms cyclic tetramers (Se03)4, in which each Se atom connects two bridging O atoms and two terminal O atoms, with Se-Ob 177 pm and Se-Ot 155 pm (AX4 type). 

The solid-state structure of Te03 is a three-dimensional framework, in which Te(VI) forms TeC 6 octahedra (AX6 type) sharing all vertices. 

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