Square-antiprismatic clusters

The more molecular character of 9 in comparison to 8-gallium is responsible for a decrease of the bond lengths by about 25 pm. These results prove that 9, considering its synthesis as well as the electronic structure, has to be classified in between polyhedral gallium clusters such as the square antiprismatic cluster GajjiCnHcih2- 72 and metalloid clusters. 

The polystannide cluster Sn94 (or the related Pb94 ) reacts with Cr(CO)3 (mes) to form a bicapped square-antiprismatic cluster [Sn9Cr(CO)3 ]4 ... 

The cluster dianions [Co8C(CO)ig]- (Fig. 8a) and [NigC(CO)i6] provide rare examples of a first-row main-group atom occupying a square antiprismatic cluster cavity,and also illustrate the capacity of carbon to become eight-coordinate. However, on progressing to the heavier main-group elements such as Si, P, S and As we find a propensity for clusters based on the square-antiprism. 

The clusters [RugP(CO)i9( - 7 / -CHjCsHs)], " [Rh9P(CO)2i] - (Fig. 8b), and [RhioP(CO)22] , provide examples of P-atoms encapsulated in un-, mono-and Z /-capped square antiprismatic geometries, respectively. The replacement of phosphorus by arsenic in the cluster [RhioAs(CO)22] (Fig. 8c),I shows the capacity of the basic square antiprismatic cluster polyhedron to accommodate interstitial atoms of this size. Elongation of the interplanar Rh-Rh contacts results, however, suggesting that the increase in the steric demands of the central atom is accommodated, at least in part, by expansion of the metal polyhedron. 

Fig. 23. The Ci, component of the e non-bonding orbitals in a capped square antiprismatic cluster...

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... 

Figure 13.27 The structures of cationic clusters of Bim"+. The dimensions cited for Bi9 + were obtained from an X-ray study on [(Bi9 +)(Bi )(HfCl6 )3] the corresponding average distances for Bi9 + in BiCli 1 7 i.e. [(Bi9 +)2(BiCl5 )4(Bi2Cl8 )] are 310, 320 and 380pm respectively. The square antiprismatic structure of Big " was established by an X-ray study of Bi8[AlCl4]2. ...

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. 

The cluster Sng 4 has a monocapped square antiprismatic geometry. On the other hand, the cluster SM5 ]2, 68b, has a trigonal bipyramidal geometry, as shown by X-ray crystallography107. 

The dianionic Ga8 cluster [Ga8(flu)8]2 453 (flu = fluorenyl) has been synthesized by salt metathesis reaction between a metastable Ga Br solution and fluorenyllithium.344 The eight Ga atoms form a square antiprismatic core. This is rather unexpected because 453, which is a z/oro-cluster according to the Wade-Mingos rules, was expected to adopt the dodecahedral structure as was observed for the corresponding boron clusters. However, computational calculations demonstrated that in the case of eight Ga atoms, the formation of a square antiprismatic structure is energetically favored (Figure 33). 

Figure 1. The structure of c o%o-[(PMe2Ph)2(l-NiBgFlyCl2-2,4)] showing the bi-capped-square antiprismatic arrangement of the NiB cluster, which is analo-...

This consideration also applies to 8-vertex clusters with interstitial atoms. The most spherical 8-vertex deltahedron, namely the bisdisphenoid (Eig. 1), appears to have too small a cavity for an interstitial transition metal. Plowever, the square antiprism has two fewer edges and can be partially flattened to make a puckered eight-membered ring, which can accommodate a transition metal in the center (Pig. 8). Known clusters of this type include M E8" (M = Cr [98], Mo [98], Nb [99] E = As, Sb n = 2,3 for Cr and Mo = 3 for Nb). The transition metal in such structures can be considered to be eight-coordinate with flattened square antiprismatic coordination. The Eg ring (E = As, Sb) can be considered formally to be an octaanion, isoelectronic with the common form of elemental sulfur, Sg. Thus in M Eg (M = Cr, Mo E = As, Sb), the central transition metal has the formal oxidation state of +6. Similarly in Nb Eg , the central niobium atom has its d formal oxidation state of +5. 

Finally, cobalt carbide-carbonyl clusters have recently been isolated through a two-step synthesis. First of all, the well-known Co3(CO)9CCl is prepared from Co2(CO)8 and CCh, and then the hexanuclear carbide dianion [Co6(CO)i5C]2- is obtained in good yields (9) by further reaction with Xa[(, o(CO)4] in diisopropylether [see Eqs. (18) and (19)]. Further redox condensation between [Oo6(CO)i5C]2-and Co4(CO)i2 [see Eq. (11)] gives the square antiprismatic octanuclear cluster [Cdo8(Cdt )i8 C]2- (13). Both these carbide derivatives, as well as all of the other cobalt high nuclearity clusters, are sensitive to air and react with carbon monoxide at atmospheric pressure. 

For Rh, the size match with P is better and square antiprismatic cavities are seen for [Rh9(CO)2ip]2" and [Rhi0(CO)22P]3 . Both of these clusters possess 11 skeletal pairs of electrons and obey Wade s rule for nido- and c/oso-clusters, respectively. Arsenic analogues of these compounds are known. 

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. 

No Bi2Xi compound is known, but it has long been known that when metallic bismuth is dissolved in molten BiCl3 a black solid of approximate composition BiCl can be obtained. This solid is Bi24Cl28, and it has an elaborate constitution, consisting of four BiCll", one Bi2Clg, and two Bi + ions, the structures of which are depicted in Fig. 10-5. The electronic structure of the Bi + ion, a metal atom cluster, is best understood in terms of delocalized molecular orbitals. Other low-valent species present in various molten salt solutions are Bi+, Bi3+, Bi +, and Bi +. The last, in Bi8(AlCl4)2, has a square antiprismatic structure. 

Compounds such as hypercloso-2 and 12 have provided an entry into the long debated question of the relationship between the structures shown in Fig. 3 and the bicapped square antiprisinatic structure for 10-vertex clusters.23 Extended Hiickel MO calculations on a model system showed that the bicapped square antiprismatic structure is preferred for the 2n+2 skeletal electron system (closo) while the C3v structure was preferred for 2n electron systems (hypercloso).23 25 Thus, the discussion has revolved around the description of compounds 2 and 12 as either hypercloso 2n electron systems23 or as isomers of closo (isocloso) 2n+2 electron systems which apparently violate Wade s rules.24b While simple redox processes would be thought to interconvert these structures, this has not been experimentally shown for underivatized species. As shown in Figure 4, we have now observed, however, the reversible and... 

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