Framework bonding

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

Symmetry of orbitals on the B6 octahedron, (a) Six outward-pointing (sp) orbitals used for a bonding to 6 H. (b) Six inward-pointing (sp) orbitals used to form the fi framework bonding molecular orbital, (c) ComponenLS for one of the fiM framework bonding molecular orbitals — the other two molecular orbitals are in the yz and zr planes, (d) (Components ftM one of the t2 framework bonding molecular orbitals — the other two molecular orbitals are in the yz and x planes. 

As each B atom contributes 1 electron to its B-Ht bond and 2 electrons to the framework MOs, the (n + 1) framework bonding MOs are just filled by the 2n electrons from nB atoms and the 2 electrons from the anionic charge. Further, it is possible (conceptually) to remove a BHt group and replace it by 2 electrons to compensate for the 2 electrons contributed by the BHi group to the MOs. Electroneutrality can then be achieved by adding the appropriate number of protons this does not alter the number of electrons in the system and hence all bonding MOs remain just filled. 

Ms " clusters have 12 framework bonding electrons as has [BsHs]- (p. 161) the anions are also isoelectronic with the well-known cation [Bis]. Similarly, the alloy NaSn. 2.23 reacts with cryptand in ethylenediamine to give dark-red crystals of [Na(ciypt)]4 [Sng] the anion is the first example of a C41, unicapped Archi-median antiprism (Fig. 10. lOc) and differs from the >3/, structure of the isoelectronic cation [Bis] + which, in the salt Bi+[Bi9] +[HfCl6]5 (p. 591), features a tricapped trigonal prism, as in [BgHg] " (p. 153). The emerald green species [Pb9] , which is stable in liquid NH3 solution, has not so far proved amenable to isolation via ciyptand-complexed cations. 

Parent carboranes are composed of carbon, boron and hydrogen atoms. Whenever a hydrogen bridge occurs (BHB or CHB), the electron of the bridging hydrogen atom is needed for the framework bonding of the cluster. Replacement of the... 

For a B Hm borane based on an anion B H 6-, an arachno-structure is preferred with B at n vertices of an (n + 2) vertices polyhedron (n + 3) pairs of framework bonding electrons are required. 

In an amazing reaction, dissolving metal reduction of dichloro diester 859 generates dianion 860 which can be conventionally methylated (Scheme CIII) Irradiation of keto ester 861 introduces yet another framework bond. Dehydration of the newly... 

When the olefinic precursor to 866 was treated briefly with trifluoromethanesulfonic acid in dichloromethane solution, cyclization occurs with installation of the final dodecahedrane framework bond. The predominant product proved to be 557 in which methyl group migration has also taken pla( °l The symmetry of this first dodecahedrane was apparent from its spectral properties and nicely detailed... 

Twelve of these electrons per rhodium (a total of 72) will be used for nonframework bonding leaving 14 for framework bonding. Thus there are seven bonding pairs in the... 

Fig, 15, Structure of the four-membered dimetallacycle [ i-bis(trifluoromethyl)acetyl-ene]bis[carbonyl(r)5-cyclopentadienyl)rhodium](/ /i—Rh) (126a) (244), a typical example of a strictly planar M2C2 framework. Bond lengths in picometers. 

Another way to count the electrons in clusters restricts the count to only those associated with cluster bonding hence, skeletal-electron pairs (sep). For three-connect clusters sep = 3/2n. The assumption that orbitals associated with cluster ligand bonding can be considered independently of orbitals associated with cluster framework bonding results in an important simplification for more complex systems. However, it is a separation that is convenient rather than rigorous. We will see... 

The conjugate base of a cluster generated by deprotonation (see Section 5.1) is a Lewis base. Any metal-metal, metal boron, or boron boron bond is a potential Lewis base. The former have considerable nucleophilic character if anionic and effectively interact with electrophiles more complex than the proton. As the site of basicity is associated with framework bonding, this reaction type results in cluster building, for... 

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