Clusters valence electrons

Interestingly, compounds of Class A or Class B have 50 cluster valence electrons if the three metal atoms are individually to have 18-electron configurations. Several of the compounds meet this requirement, e.g. (25), (27), (31) (34) or (36). However, compound (23) with 46 clusterValence ... 

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). 

The relationship between the trigonal distortion of the octahedra of Mo6 in the Chevrel phases and the oxidation states (number of cluster valence electrons) of Mo6Eg, the difference of chalcogens, the oxidation states, and the sizes of M have been repeatedly discussed (84). The main issues can be summarized as follows. 

The average oxidation state of the metals in both molybdenum and tungsten clusters is +3.5, and the number of cluster valence electrons is 10. The number is consistent with five M—M single bonds. 

The three hexanuclear clusters are mixed valency complexes of two M(III) and four M(IV) centers, and the average oxidation state of the metal is +3.67. The number of cluster valence electrons is 14, which agrees with seven M—M bonds. Relatively short M-M distances for the intra- and intertriangles suggest that the clusters can be viewed as dimers of triangular clusters. Extension of the triangle condensation would lead to higher polymers. 

The structure of 11a has been established by X-ray diffraction. The compounds 11 are 30-cluster valence-electron (CVE) dimetal species, and are therefore electronically unsaturated. The electron distribution within the W(fi-C)fth rings may be represented by various canonical forms. However, that shown, which implies that the W=C bond in the ( 5-C2B9H9R 2)(OC)2-W=CC6H4Me-4 moiety formally contributes three electrons to the rhodium center (16-electron valence shell), accords with other results from our laboratory involving electronically unsaturated dimetal compounds with bridging alkylidyne ligands 18). 

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 cluster valence electron (c.v.e.) count usually corresponds to 12 + 22 electrons. Bonding of the C2 unit involves stabilization of a, a, and ji orbitals by interaction with radial metal MOs of the same symmetry, together with overlap of ji orbitals with filled metal MOs, i.e., a similar synergic interaction to the familiar bonding mode found in alkyne-metal complexes. For the model [Co8(C2)(n-L)(L)8]4 based on two trigonal... 

Table 3.1. Cluster valence electron counts for representative metal-cluster geometries...

Exercise 4.5. Count the skeletal electron pairs (or cluster valence electrons) in the organometallic complex shown below at the left. Is there a relationship to cyclobutadiene iron tricarbonyl shown next to it ... 

Exercise 4.8. Consider the WFe2 complex at the bottom of Figure 4.19 as a trinuclear metal cluster and show that the number of cluster valence electrons = 48. 

The number of cluster valence electrons is 66. Based on the examples of metalloid clusters in the text, we don t expect the borane paradigm to work for this closed cluster and it does not (An + 2 = 58 eve). It can be viewed as a hexacapped three-connect (highly distorted) cube giving, via the cluster fusion rule 5 x 8 + 6 x 24 — 6 x 24 [or — 6 x 22] = 40 [or 52] eve. This doesn t work and besides, four of the cube edges have very long Sn-Sn contacts. Finally, it can be viewed with the modified Schleyer approach (no lone pairs on Sn atoms) or the Schleyer approach... 

For the mononuclear and dinuclear compounds, except V(CO)6, the mnnber of valence electrons per metal atom is 18. The EAN (effective atomic number) of the metal is the number of electrons of the d" configuration added to twice the number of CO groups, one electron being further added in the case of metal-metal bonded dinuclear systems. For trinuclear, tetranuclear, and hexanuclear compoimds (metal-carbonyl clusters), the cluster valence electron counting is 48, 60, and 86, respectively. As the [K(cryptand 2.2.2)] derivative. As the tetraalkylammonium derivative. As the tetrachloroaluminate. 

Relationship between geometry, molecular orbilols, ond cluster valence electrons ... 

Vertices in Parent Polyhedron Classification Boron Atoms in Cluster Valence Electrons Framework Electron Pairs Examples Formally Derived From... 

Dimetal complexes in which an RC=ML moiety formally acts as a four-electron donor to a metal (M ) as in the products 45 are electronically unsaturated species with a cluster-valence-electron (c.v.e.) count of 32. The three-membered rings may be depicted in a number of ways, the most relevant of which are shown below. 

Vertices in parent polyhedron Classification Boron atoms in cluster Valence electrons Framework electron pairs Examples Formally derived from... 

S22.9 There are four relevant pieces of information provided. First of all, the fact that [Fe4(Cp)4(CO)4] is a highly coloured compound suggests that it contains metal-metal bonds. Second, the composition can be used to determine the cluster valence electron count, which can be used to predict which polyhedral structure is the likely one ... 

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