Valence-electron counting rules,

REMARKS ON THE CHEMICAL BOND FACTOR AND VALENCE-ELECTRON COUNTING RULES... 

The 8V + 6 valence electron rule has been completely substantiated by the calculated four-membered species in Table 2 [7], Boldyrev, Wang, and their collaborators presented experimental and theoretical evidence of aromaticity in the Al/ [19] Ga/" [20], In " [20] and isoelectronic heterosystems, XAl [21], The Al/" unit (14e) was found to be square planar and to possess two n electrons, thus conforming to the (An + 2)n electron counting rule for aromaticity. The n electron counting rule would be more powerful if we could predict the number of n electrons of metal atomic rings in an unequivocal manner. Our SN+6 electron rule only requires the number of valence electrons in Al/, which is easy to count. 

The location of electrons linking more than three atoms cannot be illustrated as easily. The simple, descriptive models must give way to the theoretical treatment by molecular orbital theory. With its aid, however, certain electron counting rules have been deduced for cluster compounds that set up relations between the structure and the number of valence electrons. A bridge between molecular-orbital theory and vividness is offered by the electron-localization function (cf p. 89). 

From Tsai s pioneering discoveries [25,27], we know that atomic size, electronegativity, and valence electron counts play substantial roles in the formation of QCs. These criteria are expressed by the Hume-Rothery rules [30,31]. However, three additional highlights are also important in the consideration of possible candidate systems, at least from the viewpoint of chemists. 

In both [Fe4X4(N0)4](X = S or Se) and [Fe4S2(NO)4(NCMe3)2] the total valence electron count is 60. This is the number characteristic of tetrahedral tetranuclear metal clusters, such as [Ir4(CO)12], in the Wade and Mingos skeletal-electron counting schemes (76, 77) and, furthermore, each iron atom in these clusters obeys the 18-electron rule, provided that it forms single Fe-Fe bonds to each of the other iron atoms in the tetrahedron. 

With regard to the valence electron count, this number determines whether the transition metal ion is using its full complement of valence shell orbitals— i.e., the five nd s, the (n + l)s, and the three (n + l)p s. If the valence electron count is eighteen, all of the orbitals are fully utilized in bond formation and electron pair storage, the effective atomic number (EAN) rule is fulfilled and the metal ion is said to be saturated. If it is seventeen, the metal ion is covalently unsaturated, and if it is sixteen or less, the metal ion possesses at least one vacant coordination site and is said to be coordinatively unsaturated. The importance of the valence electron count in homogeneously catalyzed reactions has been discussed by Tolman (7). 

When the number of metal atoms in a cluster increases, the geometries of the clusters become more complex, and some are often structurally better described in terms of capped or decapped polyhedra and condensed polyhedra. For example, the first and second clusters listed in Table 19.4.3 are a capped octahedron and a bicapped octahedron, respectively. Consequently, capping or decapping with a transition-metal fragment to a deltapolyhedral cluster leads to an increase or decrease in the cluster valence electron count of 12. When a transition-metal atom caps a triangular face of the cluster, it forms three M-M bonds with the vertex atoms, so according to the 18-electron rule, the cluster needs an additional 18 - 6 = 12 electrons. The parent octahedron of [Os6(CO)is]2- has g = 86, the monocapped octahedron Osy(CO)2i has g = 98, and the bicapped octahedron [Oss(CO)22]2- hasg = 110. 

The complex [PbsfMofCOA I4 has been recently reported (Yong el al., 2005) and exhibits the structure shown below. Does the compound obey the electron-counting rules for clusters If not, where might one seek an explanation of its behavior Consider explicitly tripledecker complexes as well as complications caused by separating the external cluster lone pairs from cluster bonding pairs (see Chapter 2, Problem 12). For your information, the measured distance between the Mo atoms is 3.216 A, whereas twice the covalent radius of Mo is 2.90 A, the Mo-Mo distance in [Mo2(CO)h> 2 is 3.123 A and the Mo-Mo distance in the 27-valence electron tripledecker complex CpMo(r 5-As5)MoCp is 2.764 A. 

Structure and bonding in the tellurium subhalides can be well rationalized, either employing valence and electron-counting rules, in-... 

The classification of structural types can often be done more conveniently on the basis of valence electron counts. Various schemes for relating electron counts to structures have been proposed, with most proposals based on the set of rules formulated by Wade. " The classification scheme based on these rules is summarized in Table 15-7. In this table, the number of pairs of framework bonding electrons is determined by subtracting one B—H bonding pair per boron the n + 1 remaining framework electron pairs may be used in boron-boron bonding or in bonds between boron and other hydrogen atoms. 

What is the valence electron count for the blue ion NiCI42 Why does this tetrahedral ion not obey the 18-electron rule ... 

What is the valence electron count in (CH3)3Re(=0)2 Suggest a reason why this compound does not obey the 18-electron rule. (See A. Haaland, W. Scherer et al., Organometallics, 2000,19, 22.)... 

Metallaboranes and metallacarboranes can be classified structurally by a procedure similar to that for boranes and their main group derivatives.30 In this scheme, the valence electron count of the metal-containing fragment is first determined and then compared with the requirements of the 18-electron rule. This fragment can then be considered equivalent to a BH fragment needing the same number of electrons to satisfy the octet rule. For example, a 15-electron fragment... 

One phosphorus and three chlorine atoms supply 5 + (3 x 7) = 26 valence electrons. Since P is less electronegative than Cl, it is likely to be the central atom, so the 13 pairs of electrons are distributed as shown below. In this case, each atom obeys the octet rule, Whenever it is possible to follow the octet rule without violating other electron counting rules, you should do so. 

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