Cluster compounds Closo

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. 

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). 

Wade expanded the 1971 hypothesis to incorporate metal hydrocarbon 7T complexes, electron-rich aromatic ring systems, and aspects of transition metal cluster compounds [a parallel that had previously been noted by Corbett 19) for cationic bismuth clusters]. Rudolph and Pretzer chose to emphasize the redox nature of the closo, nido, and arachno interconversions within a given size framework, and based the attendant opening of the deltahedron after reduction (diagonally downward from left to right in Fig. 1) on first- and second-order Jahn-Teller distortions 115, 123). Rudolph and Pretzer have also successfully utilized the author s approach to predict the most stable configuration of SB9H9 (1-25) 115) and other thiaboranes. 

Sn this is not so clear. The 22 valence electrons of the Sn ion could be accommodated in exact agreement with the octet rule according to the formula given in the margin. However, calculations with the electron localization function show that lone electron pairs are also present at the equatorial atoms therefore, only six electron pairs remain for the bonds. This corresponds to the number expected according to the Wade rules, as for bo-ranes ( + 1 multicenter bonds in a closo cluster with n = 5 vertices, cf. p. 144). We will deal with the bonding in such cluster compounds in Section 13.4. 

A further extension of the known isoelectronic series in the class of icosahedral species has been reported by Todd and Beer (212). Substitution of a C—H fragment in the complexes LXIV and LXV by a bare phosphorus or arsenic atom leads to the closo-cluster compounds LXVI and LXVII, which are here also described as n complexes with regard to their synthesis. Treatment of the disodium salt of the 7,8-ElCHB9H dianion (El = P, As) with a germanium(II) species results in the formation of the... 

Reaction between the arachno-[B3H8 ] anion and complex 1 gives the arachno four-vertex complex 140 in 66% yield (92,97). Treatment of 140 with closo-[Bl0Hw]2 in refluxing ethanol affords as the major product (32%) the unexpected triruthenium decaboron double-cluster compound 141, which has been analyzed by single-crystal X-ray diffraction (97). 

A theory which shows greater applicability to bonding in cluster compounds is the Polyhedral Skeletal Electron Pair Theory (PSEPT) which allows the probable structure to be deduced from the total number of skeletal bond pairs (400). Molecular orbital calculations show that a closed polyhedron with n vertex atoms is held together by a total of (n + 1) skeletal bond pairs. A nido polyhedron, with one vertex vacant, is held together by (n + 2) skeletal bond pairs, and an arachno polyhedron, with two vacant vertices, by (n + 3) skeletal bond pairs. Further, more open structures are obtainable by adding additional pairs of electrons. This discussion of these polyhedral shapes is normally confined to metal atoms, but it is possible to consider an alkyne, RC=CR, either as an external ligand or as a source of two skeletal CR units. So that, for example, the cluster skeleton in the complex Co4(CO)10(RCCR), shown in Fig. 16, may be considered as a nido trigonal bipyramid (a butterfly cluster) with a coordinated alkyne or as a closo octahedron with two carbon atoms in the core. 

For a metal with eight valence electrons, electron precise carbonyl structures see Electron Precise Compound) are M(C0)5 and [M(CO)4]3, which are known for all three metals of the iron group, albeit with quite different properties for the different metals. The next member, [M(CO)3]6, is not formed since a four-connected vertex requires more metal-metal bonding orbitals than can be arranged on the surface of a sphere. Bonding orbitals directed towards the inside of the cluster, however, form MOs that will accommodate more electrons than would be predicted by an 18 valence electron count for each metal see Eighteen Electron Compounds), that is, 86 instead of 84 for a six-vertex closo cluster see Closo Cluster). This situation is realized in the iron group by the anions [M6(CO)i8], M = Fe, Ru, Os (isolectronic to M6(CO)i6, M = Co, Rh, Ir). 

Boranes and carboranes have structures in which their skeletal B- or C-atoms form triangular-faced polyhedra. There are basically three structural types, namely the closo- (an euphonious modification of the Greek clovo = cage, i. e., a complete or closed polyhedron), the nido (from Latin nest-like ) and the arachno- (from Greek cob-web ) structure. Each of these three types is adopted by cluster compounds of specific atomic ratios. c/o o-Structures occur in borane dianions B H , in car-borane anions (CB iH ) , and carboranes (C2B 2H ). Each skeletal atom has a single H-atom terminally attached by a bond directed outwards, away from the polyhedron center (see the example of BioHio in Fig. 3-1 below). Wo-Structures are adopted by boranes B H +4 and their related carboranes CB iH +3, C2H 2H +2 etc., and amc/z/2o-structures by boranes B H +6 and related carboranes CB iH +5, C2B 2H +4 etc. In other words, carboranes have the general formula [(CH) (BH) Hc] , where the sum a + c + x) is equal to 2 for a closo-structure, 4 for a /do-structure, and 6 for an amc/z o-structure. 

Non-Bonding Orbitals in Closo Deltahedral and Three-Connected Cluster Compounds... 

A second type of borane clusters are those with the general formula B H +4. These are classified as nido bo-ranes. Isoelectronic carboranes of the general formula C cB H +4 are produced by replacing a BH unit with a C. Nido refers to the nest-like structure of these compounds. Such a structure is obtained by the removal of one vertex from a closo structure (Fig. 22). Compounds of the general formula, B H +4 will have n electron pairs (one from each BH unit) plus four electrons or two pairs from the four additional hydrogen atoms. The total of n- -2 electron pairs will be used for skeletal bonding. Generally, any cluster compound with n -I- 2 skeletal electron pairs will adopt a nido structure. 

Phosphorus-substituted carborane ligands can be mono-, bi-, or multidentate when they react with a metal center. Factors affecting this behavior include the structural features of the carborane cage (i.e., whether it is the ortho, meta, or para isomer, or has the closo or nido structure), the substitution pattern of the carborane cluster, the number and kind of donor atoms, and the electronic and steric properties of the ligand. All these factors have an influence on the catalytic activity of the metal complex. For example, the carborane cluster can act as an electron acceptor or electron donor at the phosphorus atom [22,29]. The donor atoms adjacent to phosphorus can act as labile ligands that block the coordination site at the metal until a substrate approaches. In nido cluster compounds, the metal ion can bind in two ways through the phosphorus substituent or by the decapped face of the cluster, whereby the former option is the most stable. In nido-carborme chemistry, B-H-Rh and... 

Rhodacarborane catalysts have been immobilized by attachment to polystyrene beads with appreciable retention of catalytic activity (227). A 13-vertex /oj iJ-hydridorhodacarborane has also been synthesized and demonstrated to possess catalytic activity similar to that of the icosahedral species (228). Ak-oxidation of closo- >(2- P((Z [) 2 - i- > l[l-Bih(Z, results in a brilliant purple dimer. This compound contains two formal Rh " centers linked by a sigma bond and a pak of Rh—H—B bridge bonds. A number of similar dimer complexes have been characterized and the mechanism of dimer formation in these rhodacarborane clusters have been studied in detail (229). 

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-. 

Indium clusters have also recently been characterized, notably in intermetallic compounds. Thus, the Zintl phase, Rbzinj, (prepared by direct reaction between the two metals at I530°C) has layers of octahedral closo-lnf, clusters joined into sheets through exo bonds at four coplanar vertices. These four In atoms are therefore each bonded to five neighbouring In atoms at the comers of a square-based pyramid, whereas the remaining two (Irans) In atoms in the Ine cluster... 

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