Introduction

The Group 16 elements are usually named chalcogens (‘ore builders’), but chalcophiles in the geochemical sense are soft elements (in the terminology of the hard and soft acids and bases concept, HSAB), that preferably form minerals with sulfur and with the other heavier chalcogens selenium and tellurium. Oxygen, however, tends to bind preferably to lithophiles, which are hard elements such as the alkali metals, early transition metals, and silicon. The special position of oxygen among the chalcogen elements is associated with its very high electronegativity, which in turn correlates with its high ionization energy and the small atomic radius, and makes it difficult for oxygen to act as a central atom in electron‐rich multicentre bonds.^1–3  In contrast to the 3p orbitals of sulfur, which are appreciably larger than the 3s orbital, the 2p and 2s orbitals of oxygen have similar radial extents, because 2p shells (similarly to 1s, 3d, and 4f shells), lacking a core shell of the same angular momentum, have no radial node and are exceptionally compact.^1,3,4 

Among the non‐oxygen chalcogens covered in this Handbook of Chalcogen Chemistry, the atomic and covalent radii increase gradually from sulfur to tellurium, whereas the electronegativities of sulfur and selenium are very similar (χ=2.4–2.5, depending on the scale) and only tellurium is noticeably less electronegative (χ=2.1).⁵  The similarity of ionization energies and electronegativities of sulfur and selenium is explained by the presence in the latter of the filled 3d¹⁰ subshell. Because the 10 electrons in the 3d shell shield the 4s and 4p valence shell imperfectly from the additional 10 positive nuclear charges, the 4s and 4p ionization energies of selenium are increased to values close to those of the smaller sulfur atom. The consequence is that there is a strong similarity between S and Se isologues, whereas these differ from the corresponding O/Te derivatives. By analogy with ‘deuteration’, the term ‘selenation’ was coined to draw attention to this similarity, particularly as regards the vibrational properties.⁶  In fact, the S/Se substitution works like a quasi‐isotopic substitution, the infrared spectra of S/Se isologues being almost completely superimposable with the exception of the bands involving the S/Se atoms, with those of the selenium compounds occurring at lower frequencies with respect to those of the sulfur compounds. A practical consequence in synthetic chemistry is that a simple overlap of the infrared spectra of two S/Se compounds can be used as a diagnostic tool to ascertain their nature as isologues. Moreover, the polarization of bonds from sulfur and selenium to other elements is very similar, e.g. their nearly non‐polar bonds to carbon and iodine and the only slightly polar bonds to nitrogen. Because of the close similarity of sulfur and selenium derivatives, sulfur in minerals tends to be accompanied by trace amounts of selenium, and in organisms the trace selenium contents can be divided into a ubiquitous ‘statistical’ selenium incorporation at various sulfur sites, contrasted with the genetically programmed selenium content of essential selenoenzymes.

As an element with a stable polymeric semiconducting modification and several molecular polymorphs, selenium lies between sulfur (for which many non‐metallic polymorphs are known under standard conditions) and tellurium, which exists as a semiconducting polymer with significant interchain contacts. The ability of the chalcogen atoms to establish secondary bonds, ranging from van der Waals‐like interactions to hypervalent 3c–4e bonding systems, increases from sulfur to tellurium. When these secondary interactions involve only three chalcogen atoms (interaction of an E‐donor with an E–E acceptor pair), a great variety of linear trichalcogen systems can be observed in the solid state. As an example, a continuum of distances from very asymmetric to perfectly symmetric has been observed for E–E–E linear three‐body systems, with no indication of a critical distance at which the bond switches from a predominantly electrostatic to a substantially covalent nature.^7,8  The same observation has been made for selenium and iodine: a continuum of Se^…I contacts from van der Waals‐like interactions via ‘hypervalent’ three‐centre‐bond systems to slightly perturbed Se–I single bonds was established experimentally with help of model compounds.⁹  The correlation of increasing homonuclear or heteronuclear secondary interactions with a weakening of the neighbouring covalent bonds can be easily explained by intermolecular overlap of p‐orbitals with antibonding functions of single bonds (n→σ* interactions).^10,11 

Tellurium, which offers among the chalcogens the weakest single bonds for n→σ^* attack, has the greatest tendency to form stable species with hypervalent/hypercoordinated structures. Based on the weaker bonds, smaller HOMO–LUMO gaps (such as n/σ^*) are associated with a greater frequency of coloured compounds for the heavier elements. The significant intermolecular interactions displayed by the chalcogen elements, particularly by the heavier Se and Te, are very useful in crystal engineering for creating novel materials with extended structures, modifying the solid state electronic properties, or building functional solid state networks. In the latter respect, the synthetic chemistry developed at the tetrathiafulvalene core illustrates well how the chalcogen elements can be used in the design of electroactive materials.¹²  The remarkable variety of structural motifs found in the polymeric networks of polyselenides and polytellurides, demonstrating the much richer homopolyatomic anion chemistry for selenium and tellurium than for sulfur, can promote the development of new features such as charge density waves, low‐dimensional metallic properties, and even superconductivities in these materials.¹³  An important and rapidly growing field within the chemistry of the chalcogen elements involves chalcogen radicals, which can be isolated as stable compounds in the solid state.^14,15  Their properties are very attractive since they can be used as building blocks for the design of new magnetic materials. In recent years, the advances of nanoscience and nanotechnology have opened unexpected opportunities for chalcogenide nanomaterials, such as nanowires and quantum dots, to provide important functional nanomaterials with potential applications in optoelectronics and life sciences.^16,17 

In biological systems, detailed structures of a number of enzymes have been recently clarified thanks to the remarkable progress in the techniques of single‐crystal X‐ray analysis, and great attention has been paid to the preparation of organo‐sulfur¹⁸  and organo‐selenium compounds^19,20  as enzyme mimics. The weakness of bonds to selenium compared with those to sulfur can be an advantage when bond‐making/bond‐breaking processes are exploited enzymatically, as in antioxidant enzymes. Under physiological conditions the weaker Se–H bonds of selenols lead to significantly larger extents of protolytic dissociation than with thiols, and additionally the resulting selenolate anions RSe⁻ are softer nucleophiles than thiolates in the HSAB sense. This is also relevant to the particular role of selenium in the thyroid, catalysing a hormone‐activing deiodination step.²¹