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
