Electron precise

Its charge density distribution is like that of the cation (with sign reversal) because the added electron goes into the nonbonded orbital with a node at the central carbon atom. The probability of finding that electron precisely at the central carbon atom is zero. 

The structural principles and reaction chemistry of B-8 compounds have recently been reviewed. This includes not only electron-precise 4-, 5- and 6-membered heterocycles of the types described above, but also electron-deficient polyhedral clusters based on closo-. 

Temperature-dependent resistivity data (In p vs 1/T) for both Eu3lnP3 and Eu3ln2P4 are shown in Pig. 11.3 and indicate that they are semiconductors. The room-temperature resistivities are on the order of 1-100 cm. Band gaps were determined by fitting the data from about 130-300 K to the relationship. In p= Eg/ Ik T + f, providing a band gap. Eg, of approximately 0.5 eV for both samples. Since these two compounds can be rationalized as electron-precise Zintl phases, semiconducting behavior is expected. 

In our design, divalent Ca was chosen to partially substitute the trivalent atoms, and La and Ce were selected for a trivalent element because their ionic size (rLas+ = l.SOA rce3+=l-48A) was close to that of Ca (rca2+ = T48A) [21]. Like La, the Ce element also generally shows a formal -i-3 oxidation state in in-termetallics. Erom the reactions of the elements, we have identified as major phases the electron-precise/deficient alloys, Ln5.xCaxGe4 (Ln=La, Ce x=3.37,... 

For more electropositive elements, which have an inferior number of valence electrons in the first place, and which in addition have to supply electrons to a more electronegative partner, the number of available electrons is rather small. They can gain electrons in two ways first, as far as possible, by complexation, i.e. by the acquisition of ligands and second, by combining their own atoms with each other. This can result in the formation of clusters. A cluster is an accumulation of three or more atoms of the same element or of similar elements that are directly linked with each other. If the accumulation of atoms yields a sufficient number of electrons to allow for one electron pair for every connecting line between two adjacent atoms, then each of these lines can be taken to be a 2c2e bond just as in a common valence bond (Lewis) formula. Clusters of this kind have been called electron precise. 

Electron precise clusters with exactly one electron pair per polyhedron edge ... 

Molecules such as P4 and the polyanionic clusters such as Si4- or As2- that are discussed in Section 13.2 are representatives of electron precise closo clusters. Organic cage molecules like tetrahedrane (C4R4), prismane (C6H6), cubane (C8H8), and dodecahedrane (C20H20) also belong to this kind of cluster. 

This mode of calculation has been called the EAN rule (effective atomic number rule). It is valid for arbitrary metal clusters (closo and others) if the number of electrons is sufficient to assign one electron pair for every M-M connecting line between adjacent atoms, and if the octet rule or the 18-electron rule is fulfilled for main group elements or for transition group elements, respectively. The number of bonds b calculated in this way is a limiting value the number of polyhedron edges in the cluster can be greater than or equal to b, but never smaller. If it is equal, the cluster is electron precise. 

Mo6 octahedron) the cluster is electron-precise, the valence band is fully occupied and the compounds are semiconductors, as, for example, (Mo4Ru2)Se8 (it has two Mo atoms substituted by Ru atoms in the cluster). In PbMo6Sg there are only 22 electrons per cluster the electron holes facilitate a better electrical conductivity below 14 K it becomes a superconductor. By incorporating other elements in the cluster and by the choice of the electron-donating element A, the number of electrons in the cluster can be varied within certain limits (19 to 24 electrons for the octahedral skeleton). With the lower electron numbers the weakened cluster bonds show up in trigonally elongated octahedra. 

KT1 does not have the NaTl structure because the K+ ions are too large to fit into the interstices of the diamond-like Tl- framework. It is a cluster compound K6T16 with distorted octahedral Tig- ions. A Tig- ion could be formulated as an electron precise octahedral cluster, with 24 skeleton electrons and four 2c2e bonds per octahedron vertex. The thallium atoms then would have no lone electron pairs, the outside of the octahedron would have nearly no valence electron density, and there would be no reason for the distortion of the octahedron. Taken as a closo cluster with one lone electron pair per T1 atom, it should have two more electrons. If we assume bonding as in the B6Hg- ion (Fig. 13.11), but occupy the t2g orbitals with only four instead of six electrons, we can understand the observed compression of the octahedra as a Jahn-Teller distortion. Clusters of this kind, that have less electrons than expected according to the Wade rules, are known with gallium, indium and thallium. They are called hypoelectronic clusters their skeleton electron numbers often are 2n or 2n — 4. 

B8C18 has a dodecahedral Bg c/o.vo-skclclon with 2n = 16 electrons. In this case, the Wade rule neither can be applied, nor can it be interpreted as an electron precise cluster nor as a cluster with 3c2e bonds. B4(BF2)6 has a tetrahedral B4 skeleton with a radially bonded BF2 ligand at each vertex, but it has two more BF2 groups bonded to two tetrahedron edges. In such cases the simple electron counting rules fail. 

State which of the following clusters is electron precise, may have 3c2< bonds or fulfills the Wade rule for closo clusters. 

The electron precise bonding, as sketched in Figure 38a, was based on the structural analogy between the Te72+ structure and those of the Te72 and Te65 anions that contain similar square planar coordinated Te atoms. However, it is unlikely to invoke the presence of a dianionic Te site within the... 

In the case of the octaosmium carbonyl, it is not clear whether the heptaosmium carbonyl anion has 20 or 21 carbonyl groups in the ion. If the latter is the case, then the ion corresponds to the "electron -precise system according to or the Wade theory, and arises from an "electron-deficient neutral carbonyl species. These reactions are of considerable preparative utility for the Os6/Os5 system, and, as has been amplified in the discussion of the pentaosmium carbonyl series, form a natural... 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

Multicenter bonding is the key to understanding carboranes. Classical multicenter n bonding gives rise to electron-precise structures characteristic of Hiickel aromatics, which are planar and have 4n + 2 n electrons. Clusters are defined here as ensembles of atoms connected by non-classical multicenter bonding , i.e., all... 

Fig. 3.2-1. Electron-rich (A, B) and electron-precise (C) planar aromatics as well as three dimensional structures D—L as a result of less skeletal electrons (SE). Lines in electron-deficient corn-pounds indicate connectivities not 2c2e bonds.

Classical aromatics like the electron-rich, cyclobutadiene dianion A or cydo-pentadienyl anion B and electron-precise hydrocarbons (e.g., benzene C, Figure 3.2-1) have pure n multicenter bonds and therefore are generally not regarded as clusters. 

Heteroboranes are those in which one or more non-boron atoms replace a BH vertex, together with groups that may be attached to these heteroatoms. Boranes that contain CH vertices constitute the vast family of carbaboranes. The possibility for carbon to participate in electron-deficient frameworks contradicted the former prejudice of the always electron-precise carbon as the well-behaved brother of naughty boron. So far, most elements have been introduced as heteroatoms into borane frameworks, with the exception of the halogens and the noble gases. 

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