Cluster tetrahedral

Since the work of Dahl on synthesis and structural characterization of [Cp4Mo4S4] (89), many cubane-type complexes with Mo4S4 cores have been reported and reviewed (10,14,15). They have the general formula [Mo4S4L12] (Fig. 18), where L denotes ligating atoms with either neutral 

Main preparative routes are dimerization of Mo2 complexes and condensation of Mo3 complexes with Mo(CO)6 (10,14). Self-assembly routes from Mo(CO)6 or [MoCl3(CH3CN)3] have been developed (90). 

Reduction of [W4S6Cl2(PMe2Ph)6] (see Section IV,A) with excess Na/Hg in thf at room temperature gives [W4( t2-S)6(PMe2Ph)4] in 10% yield (94). The structure consists of an almost regular tetrahedron of W(III) atoms with each edge bridged by 2-S atom (Fig. 19). The six 

Structure of lW4S6(PMe2Ph)4]. (Reproduced from (94) with permission.) 

W-W distances (2.634(3) A) are significantly shorter than those in the rhombus cluster and consistent with 12 CVE. The W4S6 framework may be regarded as an adamantanoid core, but the coordination geometry around the tungsten atoms is distorted from a regular tetrahedron. This reductive core rearrangement of sulfide clusters is unique and will open a preparative pathway to similar clusters. 

The electronic structures of the M04E4 cubane systems have been analyzed (91, 92). Three sets of bonds, including Mo—Mo bonds. Mo—ju,3-E bonds, and Mo—L bonds, must be taken into account. The Mo—Mo bonds are weakest among the three, and they are considered separately (91). In the Mo—Mo bonding levels (a + e + t, the energy order is 2 e 1 and the HOMO is considered to be weakly Mo—Mo bonding or nonbonding (93). In the electron count, 12 CVEs (cluster valence electrons) correspond to the six Mo—Mo bonds of the cubane structures. 

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C sMc ) unit was a critical requirement. Global MW2-localized CO exchange was noted at the tetrahedral cluster lMW2Co(CO)i)(i " -C5H4Me)3, although again the Iluxional process or processes were not conclusively established. 

Slovokhotov, Yu.L. and Struchkov, Yu.T (1984) X-ray crystal structure of a distorted tetrahedral cluster in the salt [(Ph P)4Au4N] BF4 . Geometrical indication of stable electronic configurations in post-transition metal complexes and the magic number 18-e in centred gold clusters. Journal of Organometallic Chemistry, 177, 143-146. 

The 2 1 species are known as cuprates and are the most common synthetic reagents. Disubstituted Cu(I) species have the 3c 10 electronic configuration and would be expected to have linear geometry. The Cu is a center of high electron density and nucleophilicity, and in solution, lithium dimethylcuprate exists as a dimer [LiCu(CH3)2]2.3 The compound is often represented as four methyl groups attached to a tetrahedral cluster of lithium and copper atoms. However, in the presence of Lil, the compound seems to be a monomer of composition (CH3)2CuLi.4... 

Figure 8.2 Structure (C15) of cubic Laves phases. MgCu2 is the prototype. Top—Mg sub-structure with the pattern of the diamond structure. Bottom—Cu sub-structure with four tetrahedral clusters in the tetroid holes of the diamond structure. The stars indicate the centers of the patterns.

A much more subtle case of stabilisation due to the presence of face-bridging ligands is found in the two tetrahedral clusters Co4Cp4(p3-H)4 (dec. ca. 300 °C)134> and Ni4Cp4(p3-H)3 (dec. ca. 320 °C)133h The structure of the last compound is shown in Fig. 2. 

Low-valent clusters [RM]X are valuable starting reagents for various oxidation reactions as will be shown in the following for the tetrahedral cluster [TMS3CM]4 (M = Ga 383, In 384). (Reactions with metal carbonyl complexes,... 

Hirsch and co-workers calculated NICS values for tetrahedral clusters of N, P, As, Sb, and Bi, as well as for the corresponding tetra-anions composed of Si, Ge, Sn, or Pb atoms, finding diatropic values for 2,n(n 1) jr-systems.296a b It was postulated by Hirsch, Schleyer, and their co-workers that for icosahedral fullerenes and their hetero-analogues the Hiickel rule, involving 4/2+2 //-electrons, should be replaced by the 2(/2+l)2 electron rule.296... 

Fig. 1-14. Comparison of NICS for boron clusters top row, nine-vertex D tricapped trigonal prismatic clusters bottom row, four-vertex 7]j tetrahedral clusters.

The tetrahedral cluster compounds can be synthesized by three methods (1) reaction of the monohalides EX (X = Cl, Br, I) with alkyllithium or Grignard reagents, (2) reduction of suitable organoelement halides of trivalent aluminum, gallium or indium, and (3) thermolysis of R2E-ER2 compounds. 

The bonding situation of the tetrahedral clusters may be described by the qualitative MO scheme that is depicted in Figure 2.3-5 and in which the orbitals involved in the bonding of the terminal ligands are ignored [33, 35, 53], The monomeric fragments M-X have a lone electron pair in a cr-orbital as the HOMO and a degenerate set of two orbitals of re-symmetry as the lowest unoccupied state. Linear... 

Fig. 2.3-5. Qualitative MO scheme of tetrahedral clusters with elements of Group 13 and their monomeric fragments (the energy of the occupied t2 orbital depends on the degree of interaction with the empty re-orbitals see text).

The tetrahedral cluster compounds show an unprecedented chemical reactivity which led to the syntheses of a broad variety of fascinating products [59]. Some of these will be discussed in the Chapters 3.5. Only a short summary will be given here. 

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