Boron trihalide adducts donor-acceptor bond

If the donor-acceptor bond is weak, two different boron trihalide adducts of the same donor will exchange halogen to form the mixed-halogen adducts. The weaker the donor-acceptor bond, the more rapidly the mixed adducts form, indicating that free boron trihalide formed by dissociation is an active species in halogen exchange. 

The mixed boron trihalide adducts hold few surprises in terms of their donor-acceptor bond behavior, but provide striking examples of dependence of halogen redistribution behavior on the nonhalogen substituent. The simplicity and accessibility of these systems suggests their use as model compounds in the study of ligand redistribution reactions. Many of the features complicating ligand redistribution in, for example, metal carbonyl systems (46) are simplified or absent here. 

The nature of donor-acceptor complexes has been the subject of various NMR studies conducted as early as the 1960s. Early calorimetric studies showed that boron trihalides are capable of forming donor-acceptor complexes with a number of Lewis bases and the heats of adduct formation for some of these complexes were determined. Gaseous boron trifluoride, for example, was shown to form a ctxnplex with ethyl acetate in a highly exothermic reaction (-A// = 32.9 0.2 kcal mol ). IR and UV analysis of BF3 complexes of aromatic aldehydes indicated a o-complex with a lengthened CVO bond and a highly delocalized ir-system. More detailed structural information, however, was acquired only after closer inspection by low temperature H, B, C and F NMR studies. ... 

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