Clusters closo

Closo Clusters 2n + 2 Systems). The assignment of valence electrons and the factoring out of those electrons involved in exopolyhedral bonds provides 2n framework electrons for a B H molecule, two electrons short of the 2n + 2 closo count. In fact, stable neutral B H molecules are not... 

Completely closed, convex, single-shell clusters are called closo clusters their atoms form a polyhedron. If the polyhedron has only triangular faces, it is also called a delta-hedron. Depending on the number of available electrons, we can distinguish four general bonding types for closo clusters ... 

Molecules such as P4 and the polyanionic clusters such as Si4- or As2- that are discussed in Section 13.2 are representatives of electron precise closo clusters. Organic cage molecules like tetrahedrane (C4R4), prismane (C6H6), cubane (C8H8), and dodecahedrane (C20H20) also belong to this kind of cluster. 

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. 

State which of the following clusters is electron precise, may have 3c2< bonds or fulfills the Wade rule for closo clusters. 

Scheme 1). It will be noted that the irBp cluster (Figure ) has the previously unobserved C3V symmetry 1 36363 rather than the normal Dl bicapped square antiprismatic arrangement of vertices. A formal electron count requires 22 electrons (2n + 2) for the closo-cluster bonding of these 18 are supplied by the nine boron atoms, leaving U to be contributed by the Ir atom. 

Figure 10. A possible sequence of skeletal rearrangements occurring during the arachno nido — closo cluster closure reactions discussed in the text.

Species functionalized at boron, such as 50-53, have also been prepared from the corresponding MC2B4 closo clusters via the removal of the apex BH unit (decapitation) with I M Kl)A.54 57 The same approach has been employed to link CoC2B3 units to benzene or other arenes 5 As shown in the following subsection, di- and trinuclear complexes such as 53 and 54 are easily converted to multi-triple-decker sandwich systems. 

Nucleophiles react by attack at the tin center. The reaction of the trialkynyltin compounds RSnfC CR T with primary or secondary alcohols gives the alkyltin trialkoxides, RSn(ORz)3, and controlled hydrolysis gives the closo-clusters [(RSn)12(/730)i4(/r20H)6](0H)2 (Equation (86)).24 The spacer-bridged hexaalkynyldistannanes are... 

Distorted n MOs can also be found in 4c2e bonding. Expanding 2A by a CH+ unit leads to the non-planar 7r,er-distorted aromatic C2B2H4 J, isoelectronic with the (CH)42+ dication [11] G and the B4H42 dianion Q, which all can be considered as distorted tetrahedra as well as folded rings. All have ten SE and classify as closo-clusters as shown for G and J in Figure 3.2-1. 

Closo-cluster (a cage) a complete deltahedral group. 

One particularly interesting category of metallocarborane is that in which a single metal atom is shared between two polyhedra that have a vertex in eommon. In effect, the metal is sandwiched between two nido-carborane residues. Examples are shown in Fig. 17. For such commo compounds, the metal can be assumed to contribute three AO s to the skeletal bonding of each polyhedron, when the (n + 1) rule for closo clusters is found to be obeyed. For example, the isoelectronic... 

The mixed cluster Co4(CO)io(C2Et2) (50) has already been quoted (Fig. 15) as an example of a closo cluster with 6 skeletal atoms and 7 skeletal bond pairs. Nido clusters also formally based on an octahed-... 

Spiekermann A, Hoffinann SD, Fassler TF (2006) The Zintl ion [Pbio] a rare example of a homoatomic closo cluster. Angew Chem Int Ed 45 3459-3462... 

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. 

I contrast to cages like B12X12 or Cgg, clusters with similar sizes consisting of metal atoms are not stable if they are hollow the bonds at their surfaces are too weak. However, Urey can be stabilized by interstitial atoms, even if the interstitial atoms do not contribute with their electrons. Such clusters are called endohedral. Examples are the icosahedral clusters [Pt Pbi2] and [Cd Tli2] 2 with a Tl cage. The atom mentioned before the sign is the enclosed, endohedral atom. These clusters fulfill the Wade rule for closo clusters if one assumes a neutral Pt atom and a Cd ion. 

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