Cluster bicapped square antiprism

Pr )2(PPh3)7](PFg)2-3249 and [Au, i(PPlioIV1 e)i(l]3, 3250 The structure of the mixed-ligands clusters corresponds to a C3v metal skeletal framework and the homoleptic to a centered bicapped square antiprism (Figure 32). Other stoichiometries are known as, for example, [AunCl2(PPh3)8]Cl.3251... 

Alternatively, a more spherical cluster, such as a bicapped square antiprism would incorporate a C2 moiety with a C-C separation of 1.47 A and the M-M and M-C separations between 2.54 and 2.80, and ca. 1.8-2.2 A, respectively. The C2 unit is not stabilized in a square antiprismatic cavity or in other frameworks, because the inter-centroid distances are > 2.0 A, leading to C-C bond cleavage and formation of dicarbido complexes. The strength of the metal-carbide bonding is increased at the expense of the C-C bond. However, the cavity in [Nii6(C2)2(CO)23]4, is large enough to accommodate two C2 moieties. 

The deltahedron for n = 10, a bicapped square antiprism, exhibits two four-connect and eight five-connect vertices. Hence, for one heteroatom in a ten vertex c/oso-cluster we have 1 - and 2-isomers and two heteroatoms in 1,10-, 1,6-1,2-, 2,3-, 2,4-, 2,6- 2,8-isomers. Different placements generate different cluster stabilities. A rule of thumb is that the more electronegative element prefers the lower-connectivity vertex. Multiple heteroatoms more electronegative than B prefer non-adjacent positions as far apart as possible. Rearrangement to the most stable isomeric form need not be fast. In the case of icosahedral clusters, for example, the barrier to rearrangement is large and isomers can be isolated. 

Many other linked polyhedra similar to molecular boranes are found for B-rich solid compounds of alkali and alkaline-earth metals. There are octahedra in Li2B6, octahedra and pentagonal bipyramids in Na3B2o, dodecahedra and bicapped square antiprisms in U3B14 and icosahedra in Na2B29. The cluster connection is particularly clear for compounds of this type. 

Examples of thermal skeletal isomerization are as follows. The 11-electron pair cluster c/oi o-Cp2CoNiCB7Hg (Scheme 6) has a structure based upon a bicapped square antiprism (Figure 1). This polyhedron has two distinct types of vertex (i) two apical sites each of which has a connectivity of four within the cage and (ii) eight equatorial or square antiprism sites each of which has a connectivity of five within... 

Figure 2.6.1 The two known anionic clusters of tin (a) the nine-atom Sng Snf. Both clusters are deltahedral, although the nine-atom species has an open square face (the bottom). It is derived from the complete ten-atom deltahedron, a bicapped square antiprism, by removal of one of the capping vertices. The five-atom cluster is a trigonal bipyramid...

Mo and W. It is now known that both Sn " and Pbg can be derivatized with M(CO)3 where M = Cr, Mo, or W to form bicapped square antiprisms where the transition metal caps one of the squares. (It is somewhat surprising that no analogous [Ge9M(CO)3]" clusters have been characterized to date. 

Figure2.6.5 [SngCr(CO) ], the first derivative of a deltahedral Zintl anion. The Cr(CO)3 fragment caps the open pseudo-square face of the original Sn cluster and does not donate electrons for cluster bonding. The hetero-atomic cluster is a closo-species according to both shape, a bicapped square antiprism, and electron count. The three empty d orbitals, xz, andyz ofCrfCO) overlap with the filled orbitals shown in Figure 2.6.4 and, therefore, the cluster behaves as a six-electron donating ligand...

Figure 2.6.6 The isomer ri -lSiyWICO) ], a bicapped square antiprism (the pseudo four-fold axis is vertical), where the transition metal is a part of the square prism and is five-coordinate with the cluster...

The endohedral ten-vertices cluster [Ni Pbiof (Figure 3.7b) was obtained from the reaction of Ni(cod)2 with K4Pbg in ethylenediamine [69]. Herein, the Ni atom occupies the center of a bicapped square antiprism, as predicted from DET calculations [70]. Pb-NMR studies again revealed a fast atom exchange of the cluster atoms in solution, as only one (broad) signal at —996 ppm was detected [69]. 

FIGURE 37 Structure of (a) [Rhio(/i8-S)(CO)22] and (b) [Rhi7(/c9-S)2(CO)32] . The Rhio cluster is a bicapped square antiprism. A sulfide ligand occupies the square antiprism cavity. The Rhi7 cluster contains three square antiprisms. The cavity of the center one is occupied by an Rh atom. The two outer square antiprisms each encapsulate a one sulfur atom. (Terminal CO are not shown.)... 

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