Cation electron counting

Perhaps the most notable difference between S-N and N-O compounds is the existence of a wide range of cyclic compounds for the former. As indicated by the examples illustrated below, these range from four- to ten-membered ring systems and include cations and anions as well as neutral systems (1.14-1.18) (Sections 5.2-5.4). Interestingly, the most stable systems conform to the well known Htickel (4n -1- 2) r-electron rule. By using a simple electron-counting procedure (each S atom contributes two electrons and each N atom provides one electron to the r-system in these planar rings) it can be seen that stable entities include species with n = 1, 2 and 3. 

John D. Corbett once said There are many wonders still to be discovered [4]. This certainly holds generally for all the different areas and niches of early transition cluster chemistry and especially for the mixed-hahde systems. The results reported above so far cover a very Hmited selection of only chloride/iodide systems and basically boron as the interstitial. Because of the very sensitive dependence of the stable stracture built in the soHd-state reaction type on parameters like optimal bonding electron counts, number of cations present, size and type of cations (bonding requirements for the cations), metal/halide ratio, and type of halide, a much larger mixed-hahde cluster chemistry can be expected. Further developments, also in mixed-hahde systems, can be expected by using solution chemistry of molecular clusters, excised from solid-state precursors. 

Regardless of their possible metallic properties, metal-rich Zintl system or phases are defined here as cation-rich compounds exhibiting anionic moieties of metal or metalloid elements whose structures can be generally understood by applying the classical or modern electron counting rules for molecules. 

As a simple example of non-d coordination, let us consider the hexaammine-zinc(II) cation [Zn(NH3)6]2+, whose optimized structure is shown in Fig. 4.51. Each ammine ligand serves as a formal two-electron sigma donor, and the total electron count atZn therefore corresponds to a 22e system, again violating the 18-electron rule. Each ammine ligand is bound to the Zn2+ cation by about 60.7 kcal mol-1, which is in part attributable to classical electrostatic interactions of ion-dipole type. 

Whether this condition can be fulfiUed depends on the electron count of the metal, and the stereochemistry of the elimination. For instance, in m-elimination from octahedral d , or square planar d , systems, metal ndipP -y ) acts as acceptor, and this should be a facile process ( e Fip. 1, 2). For /rans-elimination, on tiie other hand, the lowest empty orbital of correct symmetry is (n + l)p. Such elimination Kerns energetically less Ukely, unless a non-concerted pathway (such as successive anionic and cationic loss) is available. The same arguments apply, of course, to oxidative additions. It foUows that the many known cases of traits oxidative addition to square planar t/ systems are unlikely to take place by a concerted mechanism, and this conclusion is now generally accepted There are special complexities in reductive elimination from trigonal systems, and these are discussed furdier in Part III. 

Possible unidentate reagents are atomic cations and other simple electrophiles as well as atomic anions and other simple nucleophiles. They can react with the cluster with or without change in the electron count as well as with or without change in cluster core shape. Not considered here are nucleophilic ligand substitutions or reactions in which the number of metal atoms in the cluster changes (cf. Section V). 

Add the number of valence shell electrons for each atom. If the compound is an anion, add the charge of the ion to the total electron count because anions have "extra" electrons. If the compound is a cation, subtract the charge of the ion (an easy way to remember is that the number of valence electrons for groups 1 7 2 is the group number, e.g.,, H-1,Ca-2,etc. and for groups 13-18 it is group number minus 1, e.g AI-13, valence electron number is 3). 

The nitrosonium cation [NO]+ is isoelectronic with CO and accordingly many mixed nitrosyl-carbonyl complexes are known. For electron counting purposes, the neutral molecule is considered to act as a 3 (or occasionally 1) VE donor. Thus various series of isoelectronic complexes can be envisaged (Table 3.5). The majority of synthetic routes to nitrosyl-carbonyl complexes involve (i) photochemical CO substitution or metal-metal bond cleavage by NO (ii) electrophilic attack by nitrosonium salts, e.g. [NO]BF4 or nitrosyl halides (e.g. C1NO) upon electron-... 

C2 units are also found in solid-state compounds with C-C separations that depend on formal electron count. These are viewed as deprotonated ethyne, ethylene or ethane using a popular solid-state idea the Zintl-Klemm concept. This concept is based on the simple idea that the metals transfer their valence electrons to the non-metal atoms thereby generating filled anion-centered bands at low energy, well separated from empty cation-based bands. Of course, this concept fails when the electronegativities of the metal and non-metal are not very different,... 

Y3+ or La3+ cations. Consequently, the additional electron populates an antibonding Trg orbital thereby reducing the bond order and increasing the C-C distance. Reality is a little more complicated since there are low-lying d orbitals on these metals. Consequently a metallic d band overlaps with the C2 TTg band as shown below. For this reason, the electron is partly localized in the metal d band and partly localized in the C2 band. Nevertheless the C-C distance in these materials is chiefly governed by the electron count of the metal. 

Many of the carbonyl cluster species are anions (cations being virtually unknown), hydrido species, or both. The relationships of these to each other and to neutral clusters in terms of electron count are the same as in simpler metal carbonyls, namely, one CO can be replaced by two hydrogen atoms, one H and one negative charge, or two negative charges. Protonation or deprotonation reactions are usually... 

The stoichiometric pyrochlore transition metal oxides exhibit a wide range of magnetic and electronic transport properties. These properties are, of course, dependent on the d electron count of the B cation. Electrical conductivity may be insulating... 

In some cases, secondary forces may exert a sizable influence on the coordination environment as well. For example, it was seen earlier how the second-order JT effect frequently manifests itself as a displacement of transition metals from the center of an octahedron. The phenomenon is only observed for metals with low d electron counts. This could be used advantageously in synthetic strategies where the goal is to selectively place cations in specific sites within a stmcture. 

Figure 3.27. The transition metal cations in the outer octahedral layers of Na2La2Ti3 xRUxOio are displaced from the centers of their octahedra. Only transition metal cations with a low d electron count (i.e. Ti +, d°) can readily accommodate this distortion. Hence, the Ru + cations (d ) are found mostly in the central undistorted layer.

Some solid-state structures are shown in Figure 10. The M4 + cations have a total electron count of 22e or 11 pairs. Using Wade s Rules, a planar cation can be derived from an octahedron by removal of two opposite vertices and may be described as iso-arachno. Support for a planar structure with extra electron density in the ring are the slightly short M-M bonds in M4 + (e.g. in 84 +, S-S = 1.98 A, vs. 2.04 A in Sg). 

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