Exo bonding

Exo Bond. A bond which is attached to a ring directly. 

Indium clusters have also recently been characterized, notably in intermetallic compounds. Thus, the Zintl phase, Rbzinj, (prepared by direct reaction between the two metals at I530°C) has layers of octahedral closo-lnf, clusters joined into sheets through exo bonds at four coplanar vertices. These four In atoms are therefore each bonded to five neighbouring In atoms at the comers of a square-based pyramid, whereas the remaining two (Irans) In atoms in the Ine cluster... 

Of course, the chemistry of zirconium cluster phases has been well described and reviewed in the literature [1-4]. Apart from a very few examples, mostly in the binary halides, almost all reduced zirconium halides contain octahedra of zirconium atoms centred on an interstitial atom Z. Several possible and experimentally realized Z include H, Be-N, K, Al-P, and the transition metals Mn-Ni. All these compounds have the general formula Ax"[(Zr6Z)Xi2X[J], with a " = alkali or alkaline earth metal cation, X=C1 Br or I, X =inner edge-bridging halide [5], X =outer exo-bonded halide, and 0[Pg.61]

Exo-Bonded Substituents and Other Main Group Derivatives of the 9 Ions. 80... 

Most of the bare main group derivatives of the E9 ions were prepared several years ago by Rudolph and Corbett, whereas the exo-bonded compounds were... 

Table 5 Main group derivatives and exo-bonded derivatives of Group 14 Zintl ions ...

Further oxidation of the nine-atom clusters to formal [Ge9] leads to linear polymers oi[-Ge9-] with two covalent intercluster bonds (Fig. 2i). Trimers [Ge9=Ge9=Ge9] (Fig. 6a) and tetramers [Ge9=Ge9=Ge9=Ge9] (Fig. 6b) occur via nonclassical bond formation between two neighboring atoms of the triangular prism basis planes of the c/oio-shaped clusters, which results in Ge-Ge-Ge bond angles of 90° and in considerably longer Ge-Ge contacts between the cluster units. Quantum-chemical calculations have shown that the exo-bonds participate in a delocalized electronic system that comprises the whole anion [204]. 

From these numbers we can see that the n electrons are much more heavily localized in the exo bonds than in the ring bonds. 

At the simplest level of Mills-Nixon-type argumentation, annelation of benzene by small saturated rings should favor the Kekule structure that places the double bond outside (exo) of the strained small ring. Thus, angular strain is expected to lead to normal bond alternation, AR > 0, where the endo bond, Ri, is longer than the exo bond, R2, as shown in 24, Scheme 26. 

When Sil4 and NaCo(CO)4 are warmed and irradiated together in /i-hexane, the cluster compound (OC)4CoSiCo3(CO)9 (entry 38) is produced, probably through the intermediacy of ISiCo3(CO)9. Its structure is shown in formula (LII) and the differing Si-Co(cluster) and Si-Co(exo) bond lengths should be noted. 

TABLE 3.28 Length (A) of the Endo and Exo Bonds of the Central Benzene Ring and Their Difference (A/ ) in 66-67 and 70-73... 

P2 and 82 In CuPS, a 1-D chain of twofold-connected P atoms with exo-bonded sulfur atoms occurs. The polymeric (PS) anion is isosteric with the as yet unknown phosphorus monochloride. 

The valence bond description, originating with Forster and refined by Coulson and Moffitt, provides a useful model of the bonding in cyclopropane. In this approach two sets of hybridized orbitals are used, one set for the endo bonds and the other set for the exo bonds. The orbitals associated with the endo bonds are calculated to be sp" hybridized and those associated with the exo bonds sp hybridized. This hybridization corresponds to a bond angle of 104° for the endo orbitals and 116° for the exo orbitals (Figure 1). 

---