Square antiprism formalism

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. 

Cationic deltahedral Pn clusters are known for Pn=Bi. [Bis], [Big], and [Bi9], which have been structurally characterized with complex counterions such as AlCLt, AsFg, and HfCle , can be obtained either by molten salt routes or by using so-called super acidic systems [205-209]. According to Wade s formalism, [Bis] " " and [Big] correspond to 12 ske closo and 22 ske arachno structures, respectively, and the latter adopts the shape of a square antiprism. [Bi9] " which is expected to form a 22 ske nido-cluster in analogy to Fig. 2d, rather adopts a distorted tricapped trigonal prismatic topology (Fig. 2c). 

Dianion [Co8(CO)i8C)] is isoelcctronic with Rh8(CO)i9C but presents a very different stereochemistry as shown in structure XX (Fig. 8) (13). Its geometry can be described as a deformed square antiprism with a carbide atom in the center of the polyhedron. This structure can be derived from that of the bicapped trigonal prism by stretching the common edge of the two capped square faces, as indicated in Fig. 8 by the dotted line. In such a process the loss of an M—M bond formally generates a square face. The idealized symmetry of a tetragonal antiprism is Did. However,... 

M formally eight coordinate (square antiprism), e.g. lanthanide(3+,4- -) and many actinide(4- -) cations alkaline-earth cations ( )... 

Interaction of XeFg with fluoride donors produces compounds formally containing [XeF7] and [XeFs]. The latter anion has been characterized in the compound [NO]2[XeFs] , by X-ray crystallography. The Xe—F distances are in the range 1.95-2.10 A and the structural arrangement is a slightly distorted, square antiprism with no definite position for the lone pair of electrons. 

In the complex rhodium carbonyl anion Rh,o(CO)22(M8-P) the P atom, (formally P ) lies at the centre of a bicapped square antiprism of metal atoms and the CO groups are attached to the comers and edges of this polyhedron [12] (8.244a). The P atom is believed to stabilise the complex and the Rh-P distances indicate it is covalently bound to eight of the metal atoms. The related anion Rh9(CO)2i([t8-P) has a similar structure with an apex Rh atom and a CO group missing. 

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