Triple bridges and cage compounds

Problem 16.7 Compare the bridging Al-O bond distances in II with the average of the terminal (covalent) bond distance and the dative O Al distance from Table 16.2. Make a rough estimate of a dative Cl— -Al bond distance. 

Problem 16.8 Discuss the bonding in the hexamer in terms of dative and covalent bonding contributions. Is the Al-N bond distance consistent with a description as 1/3 dative and 2/3 normal  

Borazine, N3B3H6, is isoelectronic and isostructural with benzene. See Section 14.4. The NB bond distance, 144 pm, is intermediate between the bond distances in the planar and orthogonal forms of aminoborane. The bonding may be described in terms of molecular orbitals similar to those of benzene each N or B atom may be regarded as sp hybridised and to form three 2c, 2ea-bonds to its nearest neighbors. Six valence electrons occupy the three r-bonding orbitals formed by combination of the six 2p AOs as indicated in the Fig. 16.9. Compare with the corresponding orbitals in benzene. Fig. 14.6. 

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