Isoelectronic relationships

Although the examples in the previous section have drawn attention to the possibility that molecules and ions with the same number of atoms and valence electrons may have different structures, isoelectronic relationship has proved an important way of connecting molecules with similar groups of atoms. The BAN rule or the equivalent rule based on the number of electrons in the outer shells of the inert gases emphasised the following isoelectronic relationships which proved to be particularly useful for interrelating the stoichiometries and structures of inorganic salts [42 5, 106]  

It also highlighted relationships between ions which did not confirm to the BAN rule - specifically the following which had a complete d shell, but vacant s and p shells (d) or a complete s shell, but an empty p shell (e). The ions shown in (e) were described by Sidgwick as inert pair compounds [54]. 

These isoelectroiuc relationships have resulted in recent years to compounds containing alkali metal cations, e.g. Na , K , etc., and the isolation of salts of the auride anion Au . They are analogues of the hydride anion [107]. As the study of transition metal and lanthanide compounds progressed, these ideas were extended to metal ions which had half-filled shells, i.e. Mn , Eu , Tb , with each of the relevant d or f orbitals containing a single electron and all the electrons having parallel spins [108], 

In 1919 Langmuir [42, 45] noted that molecules and ions containing the same number of atoms and the same total number of electrons invariably had identical structures. He described such series as isosteric groups and Table 2 below provides specific examples. 

Isosteric and the closely related isoelectronic relationships are still widely used by inorganic chemists as an effective predictor of new molecules [109]. These isoelectronic relationships provide a good guide to the occurrence and structures of the predicted molecules, although the variation in the charges of the ions can influence their Lewis acid/base properties and their redox properties. Table 3 

---

Due to their isoelectronic relationship with Arduengo carbenes , stabilization of cyclic phosphenium ions by Ti-electron delocalization has been a matter of debate. 

Similar isolobal and isoelectronic relationships [234, 235] have recently been observed for various organometallic complexes of both Group 5 metals... 

The isoelectronic relationship has been applied to the vanadium complex with hydrotris(pyrazolyl)borato ligand, which possesses the same electronic features as the Cp ligand thus the combination of complex VCl2(NAr)Tp (158) (Ar = 2,6-diisopropylphenyl, Tp = hydrotris(3,5-dimethylpyrazolyl) borate) with MAO was reported to catalyze the polymerization of ethylene (at atmospheric pressure) and propylene (at 7 bar), giving polyethylene (14kg/mol-h, Mw = 47000, Mw/Mn = 3.0) and polypropylene (1 kg/mol h, Mw = 3800, Mw/Mn = 2.0), respectively [256]. 

Non-metallocene complexes, such as aryloxide 31 and amide 138, have also been utilized as catalyst systems for the polymerization of a-olefins. Moreover, the homogeneous olefin polymerization catalysts have been extended to metals other than those in Group 4, as described in Sect. 7. Complexes such as mono(cyclopentadienyl)mono(diene) are in isoelectronic relationship with Group 4 metallocenes and they have been found to initiate the living polymerization of ethylene. These studies will being further progress to the chemistry of homogeneous polymerization catalysts. 

Carbon has one more electron than boron, so the C—H moiety is isoelectronic with the B—H or BH2 moieties. Note that an isoelectronic relationship also exists between C and BH or B. In a formal sense it should be possible to replace a boron atom in a borune with a carbon atom (with an increase of one in positive charge) and retain an isoelectronic system. The best-studied system. C,B DH,2. is isoelectronic with [BI2H, ]3 and may be synthesized readily from decaborane and alkynes and dieihyl sulfide as solvent. 

The photochemistry of the isoelectronic series (tj6-benzene)Cr(CO)3, (17s -cyclo-pentadienyl)MN(CO)3, and (i74-cyclobutadienyl)Fe(CO)3 all involve CO photosubstitution as their principal primary photoprocess.2 ) None of these are thought to undergo substitution of the 6-electron donor w-system with high quantum efficiency as previously suspected. The similarity in reactivity shows the value of isoelectronic relationships in these complexes, but no detailed treatment of the electronic structure of d6 (arene)M(CO)3 complexes has appeared. 

The UV spectra for the 7-aminofurazano[3,4-rf]pyrimidines are very similar to those of the sulfur analogues, 7-amino[l,2,5]thiadiazolo[3,4-d]pyrimidines and also to the correspondingly substituted pteridines. This similarity suggests an isoelectronic relationship... 

We have only considered polyhapto carbocyclic ligands however, the field of aromatic heterocycles is immense and conceptually simplified by recognizing simple isoelectronic relationships between CH and various (possibly substituent bearing or charged) heteroatoms. Many such... 

Chapter 1/SOME CROSS-CUTTING TOPICS Isoelectronic Relationships... 

Since this concept is a relatively familiar one, let us simply provide a few examples that are pertinent to metal atom cluster chemistry and organometallic chemistry. Some of them derive directly from the isoelectronic relationships already noted in connection with simple metal carbonyl molecules. 

Some of these trends are exemplified by the selection of molecules and complex ions in Table 1. They have been classified by (i) the total number of valence electrons (VE), and (ii) the steric number of the central atom (SN), which is calculated by adding the number of lone-pairs to the number of bonded atoms and used for interpreting molecular geometries in the VSEPR model (see Topic C2). The species listed in Table 1 illustrate the wide variety of isoelectronic relationships that exist between the compounds formed by elements in different groups and periods. Species with SN=4 are found throughout the p block, but ones with lower steric numbers and/or multiple bonding are common only in period 2. In analogous compounds with heavier elements the coordination and... 

Hydrogen cyanide has the same isoelectronic relationship to acetylene as methylamine has to ethane and pyridine has to benzene. Accordingly, having just considered silver acetylides suggests we consider silver cyanides even though these latter compounds are at least as plausibly inorganic as they are organometallic. 

Later, in 1925, Grimm formulated the hydride displacement law , according to which the addition of hydrogen to an atom confers on the aggregate the properties of the atom of the next highest atomic number. An isoelectronic relationship exists among such aggregates which were named pseudoatoms. Thus, when a proton is added to the 0 ion in the nuclear sense, an isotope of fluorine is obtained (Fig. 13.1). 

---