IV - Isoelectronic systems

Abstract

Chemical combination frequently disguises essential resemblances. The molecules C3H4 and N20, for example, have very different chemical formulas. Yet, to judge by their shapes and by their color (in the infrared, both molecules are transparent in the visible part of the spectrum), these molecules have very similar electronic structures, sufficiently similar to be called "isoelectronic."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.1 Usually, then, they have similar heavy-atom geometries --and, by inference, similar electronic structures. The molecules H2CCCH2 (allene), H2CCO (ketene), HNCO (isocyanic acid), OCO (carbon dioxide), NNO (nitrous oxide), HNNN (hydrazoic acid), and H2CNN (diazomethane), for example, all have 22 electrons and three heavy atoms; and all have linear, heavy-atom skeletons.Similarly, the molecules (CH3)2CCH2 (isobutylene), (CH3)2CO (acetone), (H2N)2CO (urea), (HO)2CO (carbonic acid), (CH3)(HO)CO (acetic acid), F2CO (carbonyl fluoride), FN02 (nitryl fluoride), H0N02 (nitric acid), H2NN02 (nitramide), and CH3NO2 (nitromethane), all have 32 electrons and four heavy atoms; and all have planar, Y-shaped, heavy-atom skeletons.(Frequently molecules have similar heavy-atom geometries if they have the same number of heavy atoms and the same number of valence electrons. Examples are NFa and PF3 (pyramidal), S02 and 03 (bent), and XeF2 and I012-(linear).)Even the general locations of bound protons are similar in molecules that have the same number of electrons and the same number of heavy atoms. The bound protons of ketene (H2CCO), for example, lie with respect to each other and the heavy-atom skeleton as do two of the bound protons in allene (H2CCCH2). Similarly, the bound proton of isocyanic acid (HNCO) lies approximately at one of the proton positions of allene, or ketene. Even more striking, perhaps, is the prediction, based on the statement above and the geometry of methane, that ammonia should be pyramidal and water bent. Figure 1 shows how much alike are the shapes of these three molecules.2These facts may be related to each other by imagining that removal of a proton from a hydrogen-heavy-atom bond leaves behind an electron pair-an unshared pair, which possibly occupies about as much space in the heavy-atom's valence shell as did the original bonding pair-and that the remainder of the electron cloud is essentially unchanged.3 Ketene's structure, for example, may be obtained from that of allene by replacing two of the C-H bonds This study was supported by a grant from the National Science Foundation and by the grant from the University of Minnesota of a Faculty Single Quarter Leave.1 "Heavy atom" means here any atom other than hydrogen or helium, or lithium. 4 With very careful work absolute structure factors can be determined with X-rays to an accuracy of about 1%. This accuracy has been achieved, however, in only a very limited number of cases (the noble gases and a few solids such as...

Use of the Isoelectronic Principle in Teaching Chemistry*

“…Introduction of the noble gas compounds can be greatly simplified by comparison with isoelectronic structures of nearby halogen compounds. For instance, the structure of XeOz is known to be very similar to that of HIOs [2]. This is illustrated in Figure 14.…”

“…The structure of carbon dioxide provides insight for constructing some new models that might otherwise be difficult to visualize [2]. Once the structure of a compound is a bit more complicated, the number of isoelectronic molecules increases rapidly.…”

“…Each of the molecules, isoelectronic to acetone, consists of four heavy atoms in a Y-shape, with one leg of the Y being a double bond, and having a total of 32 electrons [2].…”

“…While there is no guarantee (except experience) that it will not send its practitioner off on a line of unfruitful conjectures, there always is the chance that it will lead to a search for facts that lend themselves to the formulation of empirical rules and structural principles that may further our understanding of chemical bonding [2]. "…”

“…For an "in-depth" view of the isoelectronic principle and an aid to implementing it, see the references to Bent [2] and King [4] cited here.…”