Antimony coordination geometries

The length of the coordinative bond (2.86 A) corresponds to a normal Sb-Sb single bond. The coordination geometries of the donor or acceptor antimony atoms are distorted tetrahedral for the former and pseudo trigonal bipyramidal for the latter with the iodine atoms in axial (I-Sb-I 169.71°) and the lone pair, the Me3Sb and the methyl group in equatorial positions. 

The structures of these compounds have been determined by X-ray diffraction. The coordination geometries of the phosphorus atom in PPhs and the arsenic atom in AsPhs are both trigonal bipyramidal as expected, but the coordination geometry of the antimony atom in SbPhs is approximately square pyramidal (sqp). See Fig. 15.7. The symmetry of a perfect square pyramidal SbCs fragment is C4v. The fragment possesses a fourfold symmetry axis that coincides with one of the Sb-C bonds. The C atom on the symmetry axis is referred to as apical (ap). The four symmetry equivalent C atoms are referred to as basal (ba). 

Antimony pentafluoride is tetrameric in the solid phase and trimeric in the gas phase. The trimer consists of a somewhat buckled ring of alternating Sb and bridging fluorine atoms. See Fig. 16.12. The coordination geometries of the Sb atoms are distorted octahedral. 

The coordination geometry around antimony is pentagonal bipyramidal, with a benzene molecule occupying one axial site another axial site is occupied by a chlorine atom bonded to antimony by a primary Sb-Cl bond. In the equatorial plane there are two primary (normal covalent) Sb-Cl bonds (in the range 2.340-2.378 A) whereas three other secondary Sb- -Q interactions (range 3.401-4.04 A) distort the geometry and contribute to the formation of the supramolecular architecture. The supramolecular assembly exists only in the solid state in solution or melt disassembly occurs. 

Monomeric carbene complexes with 1 1 stoichiometry have now been isolated from the reaction of 4 (R = Bu, adamantyl or 2,4,6-trimethylphenyl R = H) with lithium l,2,4-tris(trimethylsilyl)cyclo-pentadienide (72). The crystal structure of one such complex (R = Bu) revealed that there is a single cr-interaction between the lithium and the carbene center (Li-C(carbene) 1.90 A) with the cyclopentadienyl ring coordinated in an if-fashion to the lithium center. A novel hyper-valent antimonide complex has also been reported (73). Thus, the nucleophilic addition of 4 (R = Mes R = Cl) to Sb(CF3)3 resulted in the isolation of the 1 1 complex with a pseudo-trigonal bipyramidal geometry at the antimony center. 

The coordination polyhedron geometry of tin-, antimony- and germanium-capped iron(II) dioximates was analyzed in the previous section. As mentioned above, the boron-capped clathrochelates have a distorted TP structure and positive QS values, whereas tin-, antimony- and germanium-capped clathrochelates have distorted TAP geometry, preferably for complexes with electronic configuration d , and a negative QS sign. 

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