Tetrahedra bicapped

Figure 25.11 Metal frameworks of some high-nuclearity binary carbonyl and carbonylate clusters of osmium (a) Os5(CO)i6 (trigonal bipyramid) (b) Os6(CO)ig (bicapped tetrahedron, or capped trigonal bipyramid) (c) [Os6(CO)ig] (octahedron) (d) Os7(CO)2i (capped octahedron) (e) [Osg(CO)22] (bicapped octahedron) (f) [Osi7(CO)36] (3 shaded atoms cap an Osu trigonal bipyramid).

In the case of neutral systems the geometry corresponds to a bicapped tetrahedron or, in other words, to a tetrahedron which undergoes two nucleophilic coordinations. They have been observed [5] to be stable in solution. A careful H variable temperature NMR study shows that these compounds undergo an intramolecular isomerization which takes place without cleavage of Si-N bonds. The AGt of this so-called "Baylar Twist" isomerization has been estimated at between 12 and 18 kcal/mole depending on the substituents at the silicon. We can conclude that this work supports strongly hexacoordinated silicon structures as possible intermediates in the usual course of nucleophilic reactions at silicon. 

The orf/zo-substituted dibenzoates, Bu2Sn(OOCCgH3X2)2 (X = OH or Cl), show201 two types of weak additional coordination. Each carboxylate has one strong (Sn-O = 211 pm) and one weaker (Sn-O = 260 pm) bond to Sn, and the two Sn—C bonds are normal (212 pm) forming a bicapped tetrahedron overall. These six-coordinate units are then weakly linked into dimers through SnO... SnO... interactions (Sn... O = 338 pm compare van der Waals sum of 370 pm)]. The interaction is qualitatively that of (30) but minor. 

We must conclude that the marked electrochemical quasireversibility is due to the remarkable geometrical reorganization from the bicapped tetrahedron of [Os6(CO)18] to the octahedron of [Os6(CO)i8]2 occurring upon the two electron addition. 

Structural reorganization associated with successive reversible two-electron reductions of [Os6(CO)i8], as illustrated by Sch. 2, can have low energy barriers. The primary reduction proceeds without the detection of an intervening monoanion and with a small activation energy barrier for the conversion of the bicapped tetrahedron to the octahedral dianion of 8kcalmol as shown in Sch. 2 the structure of the tetraanion in Sch. 2 is deduced from theoretical calculations [36]. 

C0gC(C0)i5 14), one case of a bicapped rhombus in Mo2Ni4(CO)i4 " 333), and one case of a bicapped tetrahedron in OSg(CO)jg [72] 284). The reasons for certain structures of hexanuclear clusters have been discussed in terms of several bonding approaches and will be treated in Chapter 2.3. 

Os6(CO)l8(CNC6H4Me-p)2 (62) have shown the former to have the same Os6 bicapped tetrahedron of the parent compound Os6(CO) g, whereas the latter has a rearranged metal skeleton with one isocyanide bridging three osmium atoms (40). With the ruthenium product Rus(CO),4(CNBu )2 (63), isolated... 

FIGURE 27. 1,2-shift mechanism of topomerization via bicapped tetrahedron intermediate or transition state upper part, (Ph, Cl)-interchange lower part, (O, 0)-interchange211... 

R. Mason, K. M. Thomas, and D. M. P. Mingos, Stereochemistry of Octadecacarbo-nylhexaosmium(O). A Novel Hexanuclear Complex Based on a Bicapped Tetrahedron of Metal Atoms, J. Am. Chem. Soc. 95, 3802-3804 (1973). 

This unusually high barrier for inversion of configuration at the chiral silicon atom in 70g is probably due to (0,0)-exehange via a bicapped tetrahedron intermediate, in analogy to silicon inversion in compounds 30-38.45-48... 

Exercise 3.7. Show that the eve count associated with a bicapped tetrahedron sharing an edge with another tetrahedron (eight atoms in the final cluster) is 110. 

Hexacoordinate silicon is also found in the solid state structure of 5 (Fig. 1). The coordination geometry around the silicon center is that of a bicapped tetrahedron, showing Si-N distances of 268.9 and 277.2 pm, respectively. 

The hexanuclear cluster [Os6(CO)ig] (69) and the corresponding dianion [69] are notable since the structural change from the bicapped tetrahedron of 69 (170) to the octahedron of osmium atoms in [69f (171) is exactly that predicted by polyhedral skeletal electron-pair theory (172). 

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