Cluster compounds Electron counting

Triosmium clusters with electron counts of 46, 48, 49, 50, or 52 have been described (see Table I). The 49-electron system [Os3(CO)12] appears unique and has so far only been characterized by its esr spectrum (50). The 44-electron compound Os3(CO)10 has been considered as a reaction intermediate but without substantiating evidence (51). In established cases the varying... 

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. 

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 ... 

The influence of electron-count on cluster geometry has been very elegantly shown by a crystallographic study of the deep-red compound [K(ctypt)]g [Ge9]- [Ge9] .2.5en, prepared by the reaction of KGe with cryptand in ethylenediamine. [Ge9] has the C4, unicapped square-antiprismatic structure (10.10c) whereas [Ge9]- , with 2 less electrons, adopts a distorted Dit, structure which clearly derives from the tricapped trigonal prism (p. 153).The field is one of... 

The structure of cluster compounds of transition metals and the limits of applicability of the electron counting rules forpolyhedral molecules. Y. L. Slovokhotov and Y. T. Struchkov, Russ. Chem. Rev. (Engl. Transl), 1985, 54, 323 (150). 

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). 

Our work was initiated on the reduced ternary molybdenum oxides with the thought that the metal cluster electron count (MCE) should be variable for the Mo308 cluster units. Based on Cotton s previous molecular orbital treatment of such clusters (16) it appeared that MCE s from 6 to 8 could be accommodated, but it was not clear whether the seventh and eighth electrons would occupy bonding or antibonding orbitals with respect to the M-M interactions. We thus set about to determine this from structural data on suitable compounds. The attempted replacement of Zn2+ with Sc3+ to secure the compound ZntSc°Mo308 was conducted via the reaction shown in equation 1. 

N.N. Greenwood Whether your compounds are described as hyper-closo or iso-closo depends on the number of electrons assumed to be contributed by the metal atom to the cluster. If, as is generally assumed, ruthenium contributes two electrons to the cluster in compounds such as yours, then RUC2B7 has a closo 22e skeltal electron count (i.e. 2n+2) rather than a 20e hyper-closo count. The uncertainty concerning the most appropriate choice of formal oxidation state for metals in covalent compounds permeates... 

This field has developed at a rapid pace since 1968, and a wide range of heteronuclear complexes of the tri- and tetranuclear variety has been established. It will be convenient to discuss the compounds in the first instance on the basis of nuclearity and, for the tetranuclear species, to subdivide the discussion on the basis of the carbonyl stoichiometry and cluster electron count. We have excluded from the discussion the interaction with nontransition elements, such as Hg, Tl, and Cd, which form a wide range of compounds. 

One of the first published cluster compounds of the heavier group 13 elements was the closo-dodecaaluminate K2[Ali2iBui2] 54 (Figure 2.3-10) [79], which possesses an almost undistorted icosahedron of 12 aluminum atoms with short Al-Al distances (268-270 pm). Up until today, it remained the only homonuclear cluster compound of the elements aluminum to indium which, with respect to structure and cluster electron count, is completely analogous to any boronhydride (see Chapters 1.1.2, 1.1.3, 1.1.5.2, and 2.1.5.6) (in this case doso-[Bi2H12]2 ). Compound 54 was formed in small quantities by the reaction of di(isobutyl)aluminum chloride with potassium and was isolated as dark red crystals (Figure 2.3-10). 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

K4Ge4, can be described as a polyanionic compound (as a Zintl phase also) containing the ion Ge44. This tetrahedral ion can be considered a naked (that is without any ligands bounded to the vertices) tetrahedral cluster formed by a main group element (that is Ee = 5 3 = 5X4 = 20). The electron count, on the basis of the Ge valence electrons and of the ion charge results in Ee = 4 X 4 + 4 = 20. 

Wade electron counting rules borane-like cluster nomenclature. On initially studying compounds such as boranes (boron hydrides) and carboranes (or carbaboranes boron—carbon hydrides), Wade (1976) proposed a number of rules which have then been extended to several compounds and which relate the number of skeletal electrons with the structure of deltahedral clusters. A polyhedron which has only A-shaped, that is triangular, faces is also called a deltahedron. 

Ralph Rudolph made major contributions to our understanding of the structure and bonding of polyhedral cluster compounds and he had an abiding interest in developing a rationale which would enable the structure of individual compounds to be systematized and related to each other. He independently arrived at a method of counting skeletal electrons which is now generally referred to as Wade s Rules, and this has had a decisive influence on our general perceptions of polyhedral cluster compounds. Related to this was his preoccupation with the problem of heteroatoms such as sulfur, and the number of electrons which such atoms contribute to the heteroborane clusters. 

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