Structures dodecahedral

Figure 6.24 Molecular structures of (a) tetrahedral BjCU, (b) dodecahedral BgClg, and (c) tricapped trigonal pyramidal B9CI9 and B9Br9. In BgClg note that the shortest B-B distances are between two 5-coordinate B atoms, e.g. B(l)-B(2) 168 pm the longest are between two 6-coordinate B atoms, e.g. B(4)-B(6) 201 pm and intermediate distances are between one 5- and one 6-coordinate B atom. A similar trend occurs in 69019.

Figure 8.18 (a) Proposed pentagonal dodecahedral structure of Ti8C 2. (b) The same structure viewed as a Tig cube with each face capped by a C2 group, (c) An alternative structure (see text). 

Again, these forms are energetically very similar distortions from the idealized structures make it difficult to specify one or other, and the particular structure actually found must result from the interplay of many factors. [TaFg] , [ReFg] and [Zr(acac)4] are square antiprismatic, whereas [ZrFg] " and [Mo(CN)g] are dodecahedral. The nitrates [Co(N03)4] and Ti(N03)4 may both be regarded as dodecahedral, the former with some distortion. Each nitrate ion is bidentate but the 2... 

Also of interest are the octacyano complexes, (M(CN)g] (M = Mo, W), whieh are commonly prepared by oxidation of the M" analogues (using MnO,) or Ce" ) and whose structures apparently vary, aceording to the environment and counter cation, between the energetically similar square-antiprismatic and dodecahedral forms. 

Key Colour % indicates preparation but no report of colour) mp/°C (na indicates value not reported) coordination 9 ttp = tricapped trigonal prismatic 8 d = dodecahedral 8 sa = square antiprismatic 8 btp = bicapped trigonal prismatic 8,7 = mixed 8- and 7-coordination (SrBr2 structure) 7 cc = capped octahedral 7 pbp = pentagonal bipyramidal 6 o = octahedral 6 och = octahedral chain, 6 ol = octahedral layered. 

Complexes with 5-donor ligands are generally less stable and more liable to hydrolysis than those with 0-donors but can be obtained if the ligand is anionic and chelating. Diethyldithiocar-bamates [An(S2CNEt2)4] (An = Th, U, Np, Pu) are the best known and possess an almost ideal dodecahedral structure. 

Attempts to prepare related hpp complexes of tantalum were only partially successful. Reaction of 2 equivalents of either (hpplSiMes or in situ generated Li[hpp] with Tads afforded a solid that analyzed correctly for the compound (hpp)2TaCl3. X-ray structural analysis of the crystallized product identified the coordination isomer [Ta(hpp)4][TaCl6], with a distorted dodecahedral cation and a regular octahedral anion (Figure 28). °... 

The 8-coordinate species [Mo(Et2dtc)4] can be obtained by reaction of Mo(CO)e with tetraethylthiuram disulfide (1 2) in acetone under N2. The X-ray structure revealed square-antiprismatic coordination, with a crystallographic, twofold axis coinciding with the molecular pseudo 4 axis (158). The magnetism and spectra of [M(dtc)4]"" (M = Mo or W n = 0 or 1) have been interpreted in terms of dodecahedral symmetry (159). 

In other related studies,186 we determined the structures of mixed water-methanol clusters the intensity distributions of (H20)n(CH30H)JBH+ showed magic numbers at n + m = 21, 0 < m 8 due to the enhanced stabilities of the dodecahedral cage structures in the mixed clusters. Studies of the metastable dissociation of (H20)B(CH30H)mH+ (n + m < 40) provided evidence that the dissociation channels... 

The transition-metal monopnictides MPn with the MnP-type structure discussed above contain strong M-M and weak Pn-Pn bonds. Compounds richer in Pn can also be examined by XPS, such as the binary skutterudites MPn , (M = Co, Rh, Ir Pn = P, As, Sb), which contain strong Pn-Pn bonds but no M-M bonds [79,80], The cubic crystal structure consists of a network of comer-sharing M-centred octa-hedra, which are tilted to form nearly square Pnn rings creating large dodecahedral voids [81]. These voids can be filled with rare-earth atoms to form ternary variants REM Pnn (RE = rare earth M = Fe, Ru, Os Pn = P, As, Sb) (Fig. 26) [81,82], the antimonides being of interest as thermoelectric materials [83]. 

Fig. 26 Skutterudite-type structure in terms of a framework of M-centred octahedra and b cubic arrangement of M atoms with Pn4 rings and dodecahedral cages filled with RE atoms in the ternary variants. Reprinted with permission from [110]. Copyright the American Chemical Society...

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). 

S-Donor Ligands. The full account of the crystal and molecular structure of tetrakis-(AfJV-diethyldithiocarbamato)titanium(iv), [Ti(S2CNEt2)4], has been published. The difference in the Ti—= 260.6(8) and Ti—Sg = 252.2(8) pm bond lengths in the trigonal-dodecahedral structure could be due to 7r-donation... 

Sulphur, Selenium, and Tellurium Compounds.—The crystal structure of the metal-rich zirconium sulphide Zr Sj has been determined and each sulphur atom shown to be at the centre of a square antiprism of zirconium atoms. The reaction of ZrS2 with potassium in liquid ammonia has been shown to give four K ZrS2 phases (.x = 1, 0.86, 0.71, or 0.71—0.22) which differ in the nature of the co-ordination sites occupied by the potassium atoms between the ZrS2 layers. ZrCl reacts with NaS2P(OEt)2 in toluene to form [Zr S2P-(OEt)2 4], which appears to involve a trigonal-dodecahedral arrangement of sulphur atoms about the metal. ... 

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