Species (atoms, molecules, ions) are valence-isoelectronic with each other if they have the same number of valence electrons together with the same number and connectivity of atoms, but not the same total number of electrons.
Many examples of valence-isoelectronic species have been identified. Some pairs can be truly startling such as the [SnBi3]5− ion, obtainable as the potassium compound, which is valence-isoelectronic with the carbonate ion, [CO3]2− [14].
In addition, as will be discussed in Chapter 9, there are strong similarities between specific compounds in Group (n) and matching compounds in the corresponding Group (n + 10). That is, that the compounds specifically differ by a filled d10 set (and for the elements lower in the respective groups, there is also a filled f14 electron set). It is useful to define this subset of valence-isoelectronic separately. Appropriating Massey’s suggestion earlier, pseudo-isoelectronic is proposed.
Species (atoms, molecules, ions) are pseudo-isoelectronic with each other if they have the same number of valence electrons together with the same number and connectivity of atoms, but are differentiated by a d10 or f14d10 electron set.
A good example is that of the dioxo cations: and [15]. That is, the formula resemblance among the Group 6 ions continues into the pseudoisoelectronic uranium analogue.
Using the perchlorate ion as an example, Table 6.1 shows an isoelectronic ion (sulfate); a valence-isoelectronic ion (perbromate); and a pseudo-isoelectronic ion (permanganate).
Table 6.1 Examples of the different subsets of isoelectronicity for the perchlorate ion
Significance of Isoelectronic Series
There are three reasons why the study of isoelectronic series is important.
•First, it reminds us that, following covalent bond formation, an atom does not “remember” whether, as an element, it was a metal, metalloid, or nonmetal. As such, it can be part of an isoelectronic series across “boundary lines.”
•Second, it can be a means of identifying “missing” or additional members of an isoelectronic series and spur the search for synthetic means to prepare them and report their existence.
•Third, the use of isoelectronic substitution can be used to study changes in bonding characteristics.
Following from the first point, the following triad containing a 12-member ring, is an example of an unusual structure for which the nonoxygen atom can be a metal (aluminum), a metalloid (silicon), or a nonmetal (phosphorus). Described by Greenwood and Earnshaw [16], this series is [Al6O18]18–; [Si6O18]12–; and [P6O18]6– (see Figure 6.2). Sadly, there is no “S6O18” to complete the set. Sulfur proves to be the exception to the rule, with the analogous sulfur ring compound having half the number of atoms: S3O9.
Figure 6.2 The common [X6O18]n– isoelectronic ring structure.
Table 6.2 Hexafluoro-species from Group 13 to Group 17
Moody’s example earlier in the chapter of isoelectronic hexafluoro-species is a more encompassing example. Shown in Table 6.2, with more recent discoveries added, there is a commonality of formula and structure all the way from Group 13 to Group 17.
In the context of using isoelectronic series to predict additional members, Lindh et al. looked at the possibility of extending the series of second Period oxo-species: to ArO4 [17].
Valence-Isoelectronic Relationships
Though this chapter will focus mostly on “true” isoelectronicity, valence-isoelectronic relationships are also of significant interest. One example is the use in organic chemical synthesis as oxidizing agents of the permanganate ion, ruthenium(VIII) oxide, RuO4, and osmium(VIII) oxide, OsO4 [18]. All three species are valence-isoelectronic related.
An example of the comparison of bonding in valence-isoelectronic molecules is provided by a series of heterobenzenes. Ashe synthesized phosphabenzene, C5H5P; arsabenzene, C5H5As; and stibabenzene, C5H5Sb [19]. Commencing with long known perfectly aromatic pyridine, C5H5N, he showed there was a decreasing degree of aromaticity descending the series.
Isoelectronic Matrices
It is also informative to construct isoelectronic matrices. An isoelectronic matrix is one in which all species are isoelectronic and the variation along each axis is provided by a progression in Group number. Table 6.2 was such a matrix. In Table 6.3, there is an isoelectronic matrix of 2nd Period 14/10-electron species, where 14 is the total number of electrons, and 10 is the number of valence electrons. The dioxygen dication, is included for completeness, though it is placed in parenthesis as its existence is fleeting and no stable compounds have been so far synthesized [20].
Three-Atom Isoelectronic Arrays
Among the triatomic combinations of 2nd Period elements, there is a matrix of the linear two-element 22/16-electron series, XY2, where element X varies by column and element Y by row (Table 6.4). Both N2O and N2F+ fit the formula sequence; however, while the other species are symmetric, these two are asymmetric. This difference can be explained simplistically in terms of the central atom usually being of lower electronegativity. All of these species exhibit delocalized multiple-bond character, including the N2F+ ion that has some multiple-bond character in the N−F bond [21]. The “missing” ion does exist, but it is polymeric, not a multiple-bonded monomer. The linear species is known in the gas phase. In subsequent tables, transient species produced in gas-phase reactions have been excluded.
Table 6.3 An isoelectronic matrix of diatomic 2nd Period Group 14–16 species
Table 6.4 An isoelectronic matrix of triatomic XY2 2nd Period, Group 13–17 species
Of course, there are many more options if three-element combinations are included. For the 22/16 set in Table 6.4, among those species that can be added are FCN, CNO−, and even NBC4−.
Five-Atom Isoelectronic Arrays
The isoelectronic array in Table 6.5 shows the stepwise replacement of oxygen atoms by fluorine atoms. In this example, it is the 50/32-electron combinations of 3rd Period elements from Group 14 to Group 17 with oxygen and fluorine. All of the compounds are essentially isostructural, their shape based on the tetrahedron.
Table 6.5 An isoelectronic array of triatomic 3rd Period, Group 14–17 species
Sequential
