Square antiprismatic

Sohd uranium—phosphate complexes have been reported for the IV and VI oxidation states, as well as for compounds containing mixed oxidation states of U(IV) and U(VI). Only a few sohd state stmctures of U(IV) phosphates have been reported, including the metaphosphate U(P03)4, the pyrophosphate U(P202), and the orthophosphate, CaU(PO4)2. The crystal stmcture of orthorhombic CaU(POis similar to anhydrite (194). Compounds of the general formula MU2(PO4)3 have been reported for M = Li, Na, and K, but could not be obtained with the larger Rb and Cs ions (195). In the sohd state, uranium(IV) forms the triclinic metaphosphate, U(P03)4. Each uranium atom is eight-coordinate with square antiprismatic UOg units bridged by... 

Uranium pentafluoride [13775-07-0], UF, has been isolated under different conditions, leading to two different modifications, d and p. The former is a grayish white soHd, which is synthesized from the interaction of UF [7783-81-5] and HBr or by heating UF [10049-14-6] and UF to 80—100°C. The yellowish white P-modification is also obtained by reacting UF and UF, but at higher temperatures (150—200°C). The two different modifications of UF have both been stmcturaHy characterized. The a-form consists of infinite chains of octahedral UF units. The P-form has eight-coordinate uranium atoms with the fluorides in a geometry between dodecahedral and square antiprismatic. 

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. ...

XeFjl+lAsFe]- CsXcFt [NO]+2[XcF8]2- 130.5 dec > 50 C4 , square pyramidal D44, square antiprismatic Octahedral (1) (Lone-pair inactive)... 

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... 

TetrakisidithioacetatoivanadiumCIV) was originally classified as dodecahedral. Re-examination has shown that its unit cell in fact contains two independent metal sites. One is indeed dodecahedral but the other is square antiprismatic C. W. Haigh, Polyhedron 14, 2871-8 (1995). 

MFg] , capped trigonal prismatic [MFv], and even square-antiprismatic [MFg] salts can all be isolated. By contrast with the fluorides, aqueous solutions of MCI5 and MBrs (M = Mb, Ta) yield only oxochloro- and oxobromo-complexes, though the application of non-aqueous procedures allows their use as starting materials. 

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. 

The simplest of the ir-bondcd Re-C compounds is the green, paramagnetic, crystalline, therm ly unstable ReMen, w ich, after WMe, was only the second hexamethyl transition metal compound to be synthe zed 11976). It reacts with LiMe to give the unstable, pyrophoric, Lii[ReMe(,, which has a square-antiprismatic structure, and incorporation of oxygen into the coordination sphere greatly H reases the stability, wit e,ss Re CMe, which is thermally stable up to 200 C, and Re "0[Pg.1068]

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. 

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). 

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. 

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