Monocapped octahedron

The structure of the heptaosmium carbonyl Os7(CO)2i has been determined by X-ray studies, and the 13C-NMR is consistent with the same structure in solution (231). The stereochemistry of the metal polyhedron follows Wade theory and is a monocapped octahedron (see Fig. 56). As anticipated for this structure, the 13C-NMR spectrum of an isotopically enriched sample (13CO 40%) shows three different chemical shifts, with an intensity ratio of 3 5 1. This spectrum is related to that of [HOs6(CO)18], which also has an idealized C3v symmetry, and may be viewed as [Os7(CO)21] with the Os(CO)3 cap replaced by the hy-drido group. The NMR spectrum of Os7(CO)2j only begins to show evidence of structure to the three bonds at —120°, but the resolution of the 2 1 component for each group, expected on the basis of the X-ray struc-... 

Ho(dbm>3 H20 Yb(acac)3 H20 7 Distorted monocapped octahedron Monocapped octahedron 327 328... 

FBP = pentagonal bipyramid CTP = C , monocapped trigonal prism CO = monocapped octahedron DD = dodecahedron SAP = square antiprism BTP = bicapped trigonal prism. 

UCl(Me3PO)J3+ UF7f- (in K3UF7) Distorted monocapped octahedron h... 

Fig. 42. [Re CO),]1-, 34, as in its Ph4P salt (76). The Re, core comprises a monocapped octahedron of rhenium atoms, with the carbide carbon in the octahedral cavity (mean Re-C = 2.13 0.02 A). The metal-metal bonds fall into several categories. Bonds from the capping atom to the capped face of the octahedron average 2.929 A those on the capped face, 2.9S5 A those between the capped face and the opposite uncapped face alternate longer (3.017 A) and shorter (2.977 A), and those in that uncapped face, average 3.080 A. There are three.terminal carbonyls on each metal atom.

When the number of metal atoms in a cluster increases, the geometries of the clusters become more complex, and some are often structurally better described in terms of capped or decapped polyhedra and condensed polyhedra. For example, the first and second clusters listed in Table 19.4.3 are a capped octahedron and a bicapped octahedron, respectively. Consequently, capping or decapping with a transition-metal fragment to a deltapolyhedral cluster leads to an increase or decrease in the cluster valence electron count of 12. When a transition-metal atom caps a triangular face of the cluster, it forms three M-M bonds with the vertex atoms, so according to the 18-electron rule, the cluster needs an additional 18 - 6 = 12 electrons. The parent octahedron of [Os6(CO)is]2- has g = 86, the monocapped octahedron Osy(CO)2i has g = 98, and the bicapped octahedron [Oss(CO)22]2- hasg = 110. 

The dihydride Os7(CO)2oH2 also exists in two isomeric forms, which, in this case, are formed by two very different routes (Scheme 14). Protonation of [Os7(CO)2o] gives the brown isomer (118). Only the red isomer, synthesized via the addition of Os(CO)4H2 to Osg(CO),g(MeCN)2, has been structurally characterized (118). Instead of the monocapped octahedron predicted by Wade s Rules and observed for the isoelectronic Os7(CO)2i, it has a structure based on the Osg(CO)ig bicapped tetrahedral metal atoms arrangement, with an edge bridged by an Os(CO)4 unit. These two species do not interconvert. Deprotonation of both isomers yields the same dianion and the red species, once deprotonated, even singly, cannot be reformed (474). 

The geometry around the hepta-coordinated niobium atom in the Nb(02)F52 anion has been viewed as a distorted monocapped octahedron with one oxygen capping an octahedral face. The EPR spectroscopic results indicate that the unpaired electron in the Nb(02)F5 radical anion resides in the non-bonding orbital of the peroxide group this was further supported by EHMO calculations done on the precursor. 

My research has been mainly in superacids, sulfuric acid, fluorosulfuric acid and hydrogen fluoride, which led me naturally into fluorine chemistry. I didn t do a lot more on VSEPR except for trying to promote it. I was excited by the discovery of the noble-gas compounds, and it was an obvious opportunity to show that it was easy to use VSEPR to predict their structures. At one of the early conferences on noble-gas chemistry, Larry Bartell asked me to predict the structure of XeF. I believe he had already determined the structure by electron diffraction but not yet published it. I told him that it could not be octahedral, which the MO theorists believed they had proved, but was probably a distorted monocapped octahedron, which turned out to be correct. Since then, he has been a great fan of VSEPR and has done much to promote it as a useful theory. 

FIG. 3.6. 7-cootdination poly-hedia (a) monocapped octahedron, Ho(<(>CC)CHCO0)3. H2O, (b) monocapped trigonal prism, Yb(acac)3. HjO. The broken lines show the edges spanned by the chelate ligands the shaded circles represent H2O molecules. 

The crystal and molecular structure of tetrachlorobis(triphenylphosphine oxide) uranium(vi) has been determined from three-dimensional AT-ray diffraction data. The complex was found to adopt a distorted octahedral configuration with the neutral ligands co-ordinated in the cis positions. The single-crystal study of chlorohexakis-(trimethylphosphine oxide)uranium(iv) trichloride showed the uranium atom to be co-ordinated to one chlorine and six oxygen atoms. The co-ordination polyhedron was described in terms of a distorted monocapped trigonal antiprism or a distorted monocapped octahedron. 

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