Radial bonds

B8C18 has a dodecahedral Bg c/o.vo-skclclon with 2n = 16 electrons. In this case, the Wade rule neither can be applied, nor can it be interpreted as an electron precise cluster nor as a cluster with 3c2e bonds. B4(BF2)6 has a tetrahedral B4 skeleton with a radially bonded BF2 ligand at each vertex, but it has two more BF2 groups bonded to two tetrahedron edges. In such cases the simple electron counting rules fail. 

Exercise 3.14. Treat the bonding in the [Ni38Pt6(CO)48H(6, l)] - with the structure shown in Figure 3.26 as a centered radial- and tangential-bonded cluster with radial bonding alone to the outer shell of metal fragments and compare observed and calculated eve counts. 

Answer. The eve count for [Al77R2o]2 is 253 so it is an odd electron cluster. Considering the inner two clusters with full radial and tangential bonding (the smaller one with an interstitial A1 atom) yields a count of 50 + 178 = 228. If the outer AIR fragments are attached by radial bonding alone the total count is 228 + 20 x 2 = 268 which is 15 more than observed and perhaps 14 more than a probable even electron species. Hence, one would have to assume seven tangential orbitals empty. 

Treat the metal cluster [RunHlCOh 2. with known metal core geometry shown below (a) as a cluster that obeys the counting rules for main-group clusters (b) as a fused cluster structure and (c) as a radial- and surface-bonded core cluster with external cluster fragments bound by radial bonds only. Which provides a satisfactory explanation at the level of electron counting ... 

In radially bonded clusters, the bonding cluster orbital energy levels lie lower when the number of nearest neighbors per atom is maximized, and this usually corresponds to close-packed high-symmetry structures. However, in clusters that do not correspond to completely filled, P and so on, shells, it may be more favorable for the cluster to distort and open up a significant HOMO-LUMO gap. The... 

Figure 10 Schematic energy-level spectrum for a radially bonded cluster with a main group interstitial atom...

In 1969 Gilbert showed that spiro[2.2]pentane-2,3-d2 (2) underwent stereomutation with an activation energy of 51.5 kcalmol" This is some 13 kcalmol lower than that required for stereomutation of cyclopropane-d2, a result which is really not consistent with radial bond cleavage because only 8 kcal mol stabilization would be expected from conversion of a primary radical into a tertiary one (and, in fact, a tertiary cyclopropyl radical would probably be less stabilized than a tertiary butyl radical because of the strain required to attain the near-planar geometry of a tertiary radical in a cyclopropane ring). 

Table 1. Stability, size, and magnetic moments for selected central atoms in icosahedral Fe12X clusters, that have all electrons in either filled or half-filled and maximally spin-polarized icosahedral electronic subshells. The binding energy (relative to Fe12 + X) is in Hartrees and radial bond distance in Bohr.

Central atom Binding energy Radial bond distance Unpaired electrons... 

The MXf, complex may now be considered in the light of conditions (2) and (3). In a trigonal bipyramid the angle between axial and radial bonds is just 90°, that is the X-X distance is no smaller than in the octahedron. Thus (3) leads us to expect that the octahedron will bo preferred to the trigonal pyramid. An exactly similar argument shows that the octahedron will also be preferred to a square prism. Thus, except in isolated molecules or ions, we should not expect to find MXS arrangements. 

From intensity data, axial-radial bond angles (a) of 90-95° were calculated. 

Phosphoranyl radicals have been reviewed.An e.s.r. study of a single crystal under X-ray irradiation at low temperatures allowed both stereoisomers (52) and (53) of a phosphoranyl radical to be observed. The parameters showed a small excess Sj spin density in the radial bonds in comparison with the apical bond, which refines the hypothesis that sp d hybridization of phosphorus can be constructed from three radial sp ligands and two pd ligands.Radical ions obtained by the reduction of tertiary phosphines and their chalcogenides have been studied by e.s.r. spectroscopy together with adducts of phosphites with 1,3-dialkyltriazenyl radicals,2,6-di-t-butylphenoxy radicals, and various spin trapped phosphorus radicals. ... 

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