Electron-deficient multicentre bonded compounds

In addition to forming either single M—C bonds or salt-like ionic compounds, elements such as Li, Be, Mg, Al, form compounds with bridging alkyl and aryl ligands. This class of compound, which is already familiar 

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Boron.— The remarkable structural behaviour of boron, which tends to confound the newcomer, continues to receive considerable attention, although the extent of the coverage of boron compounds is perhaps reduced by the difficulties of synthesis which often exist. Since it is immediately to the left of carbon in the periodic table and only sli tly less electronegative, covalent behaviour predominates, but the electron-deficiency of simple tervalent-boron compounds leads to an extensive variety of molecules in which boron acquires electrons in various ways, particularly from nearby bonds or lone pairs. Interpretation of the structures observed frequently involves molecular-orbital calculations, which emphasize the multicentred bonding so often found. 

In the boron compounds so far discussed, electron-deficiency is relieved where possible by the formation of multicentred bonds. In boron-nitrogen compounds, however, boron atoms often share nitrogen lone pairs, as is illustrated by the existence of the well-known Lewis-add-Lewis-base adducts this effect is also demonstrated by the frequent planarity of the QNB skeleton in dimethylaminoborane derivatives. In dimethylaminodichloro-borane, (CH8)2NBCl2, all the heavy atoms are copolanar, and a short B-N bond, 1.379 0.006 A, is observed. The authors suggest that this... 

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