Figure 6.1 One of the classic exchanges in Alice Through the Looking Glass (from Ref. [1]).
Chemists have shown a strong propensity to use the Humpty Dumpty approach to the word “isoelectronic.” Often is the word used, rarely is the word defined. The Reader is left to their own intuition as to which sort of isoelectronicity the author of a specific research paper intended. In this chapter, clarity will be provided by the use of modifiers to remove the need to: “. . . make words mean so many different things.”
Historical Definitions of Isoelectronic
The earliest discussion of the isoelectronic concept seems to date from 1919. Langmuir, in his 66-page discourse on the arrangement of electrons in atoms and molecules, commented on patterns among compounds [2]. Though some of the terms he used were archaic, the discussions clearly laid the foundation of the isoelectronic concept. Langmuir employed the symbol, N, to denote the total number of electrons, while E was used to represent the outer valence electrons. He noted the resemblance of carbon monoxide to dinitrogen, and extended the similarity to hydrogen cyanide as they all have N = 14 and E = 10. Langmuir noted many other isoelectronic linkages, such as that of cyanogen, (CN)2, and dichlorine. Of specific note, he saw the predictive ability of the isoelectronic concept [3]:
For example, since phosphorus and nitrogen atoms contain the same number of electrons in their shells, the simple octet theory represented by Equation 2, indicates that nitrogen compounds corresponding to all known phosphorus compounds could exist and vice versa.
In 1933, Penney and Sutherland [4] made the isoelectronic concept the focus of their work on the shapes of triatomic species, though their definition was somewhat circuituitous: one simply counts the number of electrons in the molecule not in closed electron shells. The authors showed that all triatomics with 16 “molecular or valence” electrons, as they called them, had a linear geometry about the central atom while the 18-electron species were “wide-angled.”
Use of the term isoelectronic principle seems to have become prevalent in the 1950s and 1960s. Moody [5] used the principle to explain the identical structures of all the following highest oxidation-state fluoride species:
Moody cited Penney and Sutherland for his own definition of isoelectronic:
Simple structures containing the same number of valency electrons are represented by the same bond diagram.
Coulson, in his classic book, Valence, described the principle as being [6]:
Two molecules with the same number of valence electrons are isoelectronic: and the principle states that such systems will have similar molecular orbitals.
Brown had a narrower definition [7]:
Two molecular species with the same number of atoms and the same total number of valency electrons are said to be iso-electronic, and the isoelectronic principle states that such molecular species will have similar molecular orbitals and molecular structures.
While companion used isoelectronic in an all-encompassing manner to indicate any cluster of atoms whose total electrons added up to the same value [8]:
The noble-gas atom Ne and the molecules HF, H2O, NH3, and CH4 all have the same number of electrons (are isoelectronic).
Bent had an equally broad definition [9]:
As a general rule, or principle, molecules are isoelectronic with each other when they have the same number of electrons and the same number of heavy-atoms.
Modern Definitions of Isoelectronic
Most first-year university texts use the term “isoelectronic” (without definition) to justify the formation of ions by the main group elements. Specifically, cations are formed that are isoelectronic with the preceding noble gas; anions are formed that are isoelectronic with the following noble gas.
However, there does seem to be some convergence on isoelectronic definitions. For example, Housecroft and Sharpe [10] provided the restrictive definition of:
Two species are isoelectronic if they possess the same total number of electrons.
They continued:
The word isoelectronic is often used in the context of meaning “the same number of valence electrons,” although strictly such usage should always be qualified; e.g. HF, HCl, and HBr are isoelectronic with respect to their valence electrons.
While Massey [11] gave a similar but subtly different definition of:
Species that have identical ligands and the same number of electrons on the central atom are said to be isoelectronic and almost invariably they have the same molecular structure.
He, too, looked upon the valence electron counting as a different case:
The principle can often be extended to include species that are not strictly isoelectronic but in which the central atoms have the same number of outer electrons rather than the same total number. The and ions are pseudo-isoelectronic with tetrahedral SiCl4 and can be expected to have the same structure.
Proposed Definition
Throughout science, the prefix iso- means “the same.” Thus, strictly speaking, the term “isoelectronic” should simply mean the same number of electrons, period. This all-encompassing meaning needs to be narrowed down if isoelectronic is to have a useful role in identifying chemical patterns and trends. For true (or exact) isoelectronic status, the most logical definition would be the following:
Species (atoms, molecules, ions) are isoelectronic with each other if they have the same total number of electrons and of valence electrons together with the same number and connectivity of atoms.
This definition will be used in the following sections of this chapter. One of many more unusual examples of truly isoelectronic species are [In(NO3)4]− and [Sn(NO3)4].
Of course, a new term is needed to describe species that have the same number of valence electrons but not necessarily the same total number of electrons. Almost 50 years ago, Gillis had proposed homoelectronic [12] while Massey favored the term pseudo-isoelectronic [11]. Neither of these terms explicitly identifies their meaning. Elliott and Boldyrev [13] have used the term valence-isoelectronic. This would seem the most appropriate term as the Reader is immediately aware of the meaning without need to resort to a chemical dictionary. To provide examples for
