Square antiprismatic complexes

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

The tantalum(IV) hydrides [TaH2Cl2(PMe3)4] (51) and [TaH2Cl2(dmpe)2] (50) were characterized by low temperature X-ray crystallography. SSS,69Z Complex (51) adopts a distorted dodecahedral geometry in the solid state, while (50) is better described as a distorted square antiprismatic complex. The hydrogen atoms have been located. 

Fig. 7 Valency counting picture and the nonbonding densities for square-antiprismatic complexes. Similar to [25], s orbital contribution is omitted from the plot for clarity...

For a square-antiprismatic complex, we can approximate the coordination geometry to be cylindrical, which has a face dual as bicone (to be exact, the face dual of square antiprism is tetragonal trapezohedron, but like the pentagonal bipyramid case above, cylinder and bicone will be a good enough approximation for our purpose). A bicone resembles the density peaks of a d 2 orbital, so we can predict that the nonbonding electron densities are predominantly contributed by the electrons on a d 2 orbital. 

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... 

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. 

T. Baker, DuPont Central Research In reference to your so-called iso-closo iridaborane complex, H(PPh3)2IrB9H9, we have recently published two papers (1,2) dealing with isoelec-tronic, isostructural ten-vertex ruthenacarborane complexes and have demonstrated that these structures are related to the common closo bicapped square antiprismatic structure by the removal of two electrons (i.e. 2N skeletal electrons for an N vertex polyhedron). Such complexes have been referred to as hyper-closo to imply that the electronic unsaturation is not primarily metal-based (as in, for example, nido-(PPh3)2RhC2B8-Hx 2 (3) or closo-(PPh3)ClRh(1,7-C,BQH,) (4), but is delocal-... 

The geometry at cerium is distorted square antiprismatic with a mean stagger of -28°, a value noticeably less than that (42°) observed in the porphyrin complex. The Ce-N distances are in the range 2.411(9)-2.430(9) A, distances that are markedly shorter than those in the porphyrin structure that range between 2.467(3) and 2.483(3) A (Fig. 8). 

Complexes of the lanthanides with only one ligand of this type have been reported so far. Paetzold and Bochmann (276) have reported the complexes of lanthanide perchlorates with DMSeO which have the composition Ln(DMSe0)8(C104)3. A distorted square antiprismatic structure with a point group symmetry )4 has been proposed for these complexes. 

The polyhedron around La(III) in La(Py0)8(C104)3 has been described as a square antiprism, highly distorted toward that of the cube 149). Distorted square antiprismatic geometry has also been found in Eu(thd)3(DMF)2 283). In this complex, two of the thd moieties form one square face, and one thd and two DMF molecules form the other square face. The DMF molecules occupy cis positions on the square face. As in the case of Eu(thd)3 DMSO 283), Eu(thd)3(DMF)2 also shows two nonequivalent conformations in the same unit cell. 

The square antiprismatic or distorted square antiprismatic geometries are found in various inorganic and chelated complexes of the lanthanides and antinides. It has been suggested that the tetrafluorides of cerium (98) praseodymium (99),... 

Among the chelated species, octacoordination is often encountered. The tetrakis-acetylacetonate complex of U(IV) (a-form) and Ce(IV) are isostructural. A two-dimensional X-ray analysis showed a slightly distorted square antiprismatic geometry [109—111) for Ce(acac)4 belonging to space group P2i/c [Ctn] with an average Ce—O (acac) bond length of 2.40 A and an <0—Ce—0 of 72°. The a-form of Th(acac)4 is found to be isomorphous (770) with Ce(acac)4 (Th—0=2.41 A)... 

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