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 Isoelectronic Series
It is also possible to construct informative series in which only an individual row is isoelectronic, but successive rows are linked in a simple stepwise manner. For example, in an isoelectronic oxidation-state array, each row contains species having one more electron than the preceding row. Thus, descending the table, the oxidation state of each central atom decreases by one unit.
The array in Table 6.6 shows the 2nd Period series of oxo-species from Group 14 to Group 16. As the oxidation state of the central atom decreases, so the bond angles decrease from 180° to progressively smaller values as one, two, and three nonbonding electrons are added to the central atom. Nitrogen dioxide is one of the few stable radical species, and it is of note that the isoelectronic is stable enough to be found in biological systems [22].
In the matrices and arrays shown earlier, the number of atoms remains the same. Some interesting arrays can be created in which the number of peripheral atoms is decreased stepwise. Table 6.7 shows the second Period 10/8 hydride isoelectronic series for Group 13 to Group 17. The horizontal axis tracks the Group number while the vertical axis represents the decreasing number of hydrogen atoms. Šima has used comparisons of atomic orbital energies to examine why two “missing” members cannot exist, namely H4O2+ and HNe+ [23].
Table 6.6 Sequential isoelectronic array of triatomic 2nd Period, Group 14–16 species
Table 6.7 Sequential isoelectronic array of a 2nd Period hydride series (Group 13–17)
Table 6.8 Sequential isoelectronic arrays of chloro-species of 3rd Period, Groups 13–16
The following array (Table 6.8) shows the successive isoelectronic rows of 3rd Period main-group chloro-species as chlorine atoms are subtracted. In each column, the element is in its highest oxidation state. Vertically, the geometry changes from octahedral through trigonal
bipyramidal to tetrahedral.
A Transition Metal Array
Up to this point, all the discussions have been on arrays involving main-group elements. Arrays can be found, too, for transition metals. Among the transition metals, the “heavy” transition metals show some of the most interesting isoelectronic patterns. The array in Table 6.9 has each 5th Period early transition metal in its highest oxidation state. Descending the table, the number of fluorine atoms decreases until it matches the oxidation state.
Table 6.9 Sequential isoelectronic array of fluoro-species of 5th Period, Groups 4–7
Oxidation State as a Variable
In the preceding arrays, each of the central atoms was in their highest oxidation state. It is possible to construct arrays of isoelectronic series in which the variable is not only the number of peripheral atoms, but also the oxidation state of the central atom. This type of array can be illustrated using three successive isoelectronic series of 5th Period fluoro-compounds, stretching across from Group 13 all the way to Group 18 (Table 6.10). These species differ by one charge unit horizontally and two charge units vertically.
There is clearly a “missing” member from the array: the trifluoroxenate(II) ion, This species has indeed been sought. However, at the time of writing, the only evidence of this ion’s existence is as a transient intermediate in gas-phase studies [24].
Table 6.10 Sequential isoelectronic arrays of fluoro-species of 5th Period, Groups 13–18
Arrays of Organometallic Species
A significant proportion of organometallic species obey the 18-(valence)-electron rule [25]. Thus, it is not surprising that there are many possible isoelectronic series in this branch of chemistry. In Table 6.11, each row contains an isoelectronic series of 4th Period transition metal carbonyls with each subsequent row having one carbonyl ligand less [26].
As dinitrogen is, itself, isoelectronic with carbon monoxide, substituting dinitrogen for carbonyl ligands results in another isoelectronic series:
While stepwise substituting nitrosyl for carbonyl requires shifting from metal to metal to maintain isoelectronic status:
The isoelectronic principle has also been used to consider, as a replacement for carbon monoxide as a ligand, the potential isoelectronic species of BF and the valence-isoelectronic species of SiO [27]. However, the simple application of isoelectronicity to the trio BF, CO, N2, does not take into account the polarity and bond order changes along the series, making simple ligand replacement by BF highly unlikely [28]. Nevertheless, performing a valence-isoelectronic substitution of CO by CS as a ligand has been accomplished [29].
Table 6.11 Isoelectronically related organometallic 4th Period carbonyl species (adapted from Ref. [21])
Isoelectronicity: The Future
The isoelectronic principle continues to fascinate. For example, in fullerene research, C59N+ has been synthesized, isoelectronic with C60 [30]. A new fruitful area of isoelectronic species is high pressure, high temperature synthesis [31]. One of the early compounds to be manufactured in this category was diboron oxide, B2O. This compound has a similar structure to the isoelectronic graphite allotrope of carbon [32].
Pyykkö has been extremely active in searching for new and novel isoelectronic series. In the abstract of his review, he wrote [33]:
A combination of ab initio calculations with the isoelectronic principle and chemical intuition is a useful way to predict new species.
Pyykkö was particularly interested in the isoelectronic series of [PAuP]5−; [SAuS]3−; and [ClAuCl]−. He mused whether the series could be continued to the right. Indeed, valence-isoelectronic [XeAuXe]+ has been identified by mass spectrometry.
Commentary
Clearly, the term “isoelectronic” is a useful one but it is essential that a common definition is agreed. It does seem to make sense to provide a very narrow and unique definition of isoelectronic while valence-isoelectronic can be used for the more general term. As shown earlier, the Reader can see that true (exactly) isoelectronic series “lurk” not only across the nonmetallic elements of each period but even stretch through the metalloid members, into the weak metals. And with the options of valence-isoelectronic, and pseudo-isoelectronic, even more vistas await.
Back in 1952, Coulson ended the section on isoelectronicity with the comment [6]:
The isoelectronic principle is not now greatly used except in atomic spectra, and there are, indeed, sometimes difficulties in its application.
Coulson’s gloomy prognosis has proved to be completely wrong. Who knows what other possibilities await to be synthesized?
References
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al., “Is CO a Special Ligand in Organometallic Chemistry? Theoretical Investigation of AB, Fe(CO)4AB, and Fe(AB)5 (AB = N2, CO, BF, SiO),” Inorg. Chem. 37(5), 1080–1090 (1998).
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30.K.-C. Kim, F. Hauke, and A. Hirsch, “Synthesis of the C59N+ Carbocation. A Monomeric Azafullerene Isoelectronic to C60,” J. Am. Chem. Soc. 125, 4024–4025 (2003).
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Chapter 7
Group and Period Patterns among the Main Group Elements
As will be seen in the subsequent chapters, there are a variety of linkages among the elements. In this chapter, the focus will be upon patterns and trends within a Group or a Period. This topic, by necessity, must be limited to a few selected examples.
Whole monographs have been written on patterns and trends down groups and across periods [1–4]. As a result, this chapter on the main group elements will be very selective in the chosen examples.
Periodic Table, The: Past, Present, And Future Page 9