Square prismatic structure

Again, fluoro and oxo complexes of rhenium predominate. The reaction of KF and ReFe in an inert PTFE vessel yields pink K2[ReFs], the anion of which has a square-prismatic structure hydrolysis converts it to K[ReOFs]. 

Figure 3-29. The square prismatic structure of the IReiClsJ2 ion which played a historic role in the discovery of metal-metal multiple bonds (see, text) and a Soviet stamp with the same structure. The building in the background is the research institute in Moscow where the first such structure was prepared.

Fig. 2(/ )]. These are expected to be stable organic molecules. However, based on the problems of stereocontrol (see Section II.B) it is unlikely that this chemistry will soon start to develop. The computed stmctures indicate that the hexadentate ligand will enforce a trigonal-prismatic coordination geometry [Fig. 2(a)], and the octadentate ligand has a preference for square-prismatic structures [Fig. 2 b)]. 

The structures of the primary hydration sphere in solution are similar to those reported for the hydrated crystals (Albertson and Elding 1977), i.e. for the lighter cations n = 9 with tricapped trigonal prism (TCTP) structure and D31, symmetry, and for the heavier cations n = 8 with square prismatic structure and D2h symmetry. The geometric parameters (Ln HjO and H2O H2O distances) were taken from the X-ray data on solutions from Habenschuss and Spedding (1979b, c, 1980). 

Different bismuth clusters are apparently formed in reactions between bismuth and bismuth trichloride-aluminum trichloride mixtures, from which the salts (Big) " (AICl4 )3, and (Bi8) + (AICl4 )2 have been isolated 43). A trigonal-bipyramidal structure was predicted for Big (a closo 6 skeletal bond pair system cf. C2B3Hg), and a square anti-prismatic structure for Bi8 , as appropriate for an arachno 8-atom, 11 skeletal bond pair cluster. Similar polyhedral shapes appear likely for other clusters not only of bismuth but also of other heavy main group metals (the anion Pbg ", for example, is isoelectronic with Bi9 " ). 

Polymetallic anions, prepared by dissolution of alloys of the alkali and post-transition metals in amine solvents (often with a complexand for the alkali metal cation), have been characterized in crystalline and solution phases. Clusters TlSng3, Ge92 (with 20 skeletal bonding electrons), Sn93- (21 skeletal e) and Bi95+ (22 skeletal e) possess a tricapped trigonal prismatic structure, symmetry D3A, with variations of dimensional detail which correlate with the electron population.291 292 This structure is a ctoso-deltahedron, and with 20 (2h + 2) skeletal electrons can be construed to be three-dimensionally aromatic.292 The 22e clusters M94 (M = Ge, Sn, Pb) occur as the C4v monocapped square antiprism, a nido polyhedron. 

Fig. 43. Molecular structure of Ba(tfac)2(hmtt) showing the square prismatic geometry around the barium atom. The average Ba—Olfac and Ba—N bond distances are 2.677(9) A and 2.983(1) A, respectively. (Redrawn from Ref. 198.)... 

Molecules with multiple bonds between metal atoms often have structures with beautiful and highly symmetrical polyhedral shapes. The square prismatic [Re2Cl8]2 ion, shown in Figure 3-29, played an important role in the history of the discovery of metal-metal... 

This is particularly true now with the recent discovery of trigonal prismatic structures for some 6-coordinated metal complexes (21). Surely the proper metal-ligand combination should give a system in which the difference in energy between an octahedral and a trigonal prismatic structure is small. As a result, racemization may take place by a trigonal twist mechanism. Twisting processes of the square-planar-tetrahedral type are known to occur readily in certain nickel(II) complexes (20). 

In one of these studies (26), it was observed that Co(mnt)2 reversibly adds one mole of ligand to form the complex Co(mnt)3. The tendency of the square planar Co(mnt)2" anion to form readily 6-coor-dinate adducts with bidentate ligands and only 5-coordinate species with monodentate ligands led to the suggestion that the tris complexes might possess a trigonal-prismatic structure. 

Crystal structures of V(S2CMe)4 (containing both dodecahedral and square prismatic eight-coordinate molecules in the same crystal),337 Ni(S2CPh)2,338 Ni(S2CCH2Ph)2,339... 

Fig. 26) (230). This led to a Mn11 complex having a square prismatic geometry. The second complex is composed of a cryptate ligand that yields a complex with a cubic symmetry (Fig. 27) (231). The third structure is built up from two tetradentate tripodal amine ligands about the Mn I ions (Fig. 26) (232). 

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