Clusters valence electron deficiency

The electrostatically favored cation (Li) and anion (RE) arrangement implies the presence of two different E-, Si- and Li sorts, which has been established by solution and solid-state NMR spectroscopy. The electronic structures of the mixed-valent pnictides 10 and 11 have been simply described as electron-deficient clusters with delocalized framework electrons. Formally the latter consist of two low-valent anediyl moieties RE and eight andiides (RE)2- (E = P, As). The relatively large E-E distances of >4 A exclude the occurrence of localized E-E bonds. However, delocalization of the cluster valence electrons is achieved without Li-Li bonds via Li-mediated multiple bonding. Evidence for this has been seen in the NMR spectra (31P, 7Li, 29Si), which are in accordance with the electron delocalization model (see later discussion). 

When it comes to metal-rich compounds of the alkaline earth and alkali metals with their pronounced valence electron deficiencies it is no surprise that both principles play a dominant role. In addition, there is no capability for bonding of a ligand shell around the cluster cores. The discrete and condensed clusters of group 1 and 2 metals therefore are bare, a fact which leads to extended inter-cluster bonding and results in electronic delocalization and metallic properties for all known compounds. 

The number of cluster valence electrons (CVE) expected for a linear M3 array is 50 (110), however the formulation from the X-ray and magnetic susceptibility data suggests a 48 CVE count for the [Fe3(CO)u]2- moiety. It is possible that the Fe3(CO)u unit is electron deficient, but another explanation is that it is a 50-electron system with two hydride ligands. No evidence for an M—H bond is present in the H-NMR spectrum. Similarly, the disposition of the carbonyl ligands on the terminal iron atoms does not suggest the presence of hydrogen ligands. 

A novel and far-reaching type of isomerism concerns the possibility of valence isomerism between nonclassical (electron-deficient) clusters and classical" organoboron structures. Thus, n-vertexed /do-boranes. have cluster structures... 

Of course, valence electron concentration is not only related to the metal atoms but also to the number and valence of the ligands. Ligand deficiency creates vacant coordination sites at metal atoms and results in cluster condensation, which is the fusion of clusters via short M-M contacts into larger units ranging from zero- to three-dimensional. The chemistry of metal-rich halides of rare earth metals comprises both principles, incorporation of interstitial atoms and cluster condensation, with a vast number of examples [22, 23]. 

Polynuclear platinum and palladium carbonyl clusters containing the bulky tri-ferf-butylphosphine ligand are inherently electron-deficient at the metal centers. The trigonal bipyramidal cluster [Pt3Re2(CO)6(P Bu3)3], as shown in Fig. 11.4.4(a), is electronically unsaturated with a deficit of 10 valence electrons, as it needs 72 valence electrons to satisfy an 18-electron configuration at... 

Deformation density calculations [66] based on X-ray and neutron diffraction data indicated a substantial electron deficiency in the sulphur valence shell. Kappa refinement based on the point charge model relates a positive charge close to 1.8 to the sulphur, and about -1.0 on each of the sulphate oxygen atoms [66]. The comparison of these results to those obtained from MO cluster model calculations, provides an indication of the goodness of the applied atomic basis sets. Such a comparison is presented in Table 4. 

In contrast to the later elements, B forms a large number of so-called electron-deficient cluster compounds, the bonding in which poses problems within valence bond theory we introduce these compounds in Section 12.11. 

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