Halogen Redistribution

Halogen exchange in the coordinatively saturated boron trihalide adducts requires either a preliminary dissociative step or an associative interaction different from the doubly halogen-bridged transition state available to three-coordinate boron compounds [Eq. (1)] (113,114). The reactions can be classified as follows. 

Dissociation of the Donor-Acceptor Bond Apparently, equation 

Exchange of halogen could then proceed either through a four-center transition state (which, however, requires one boron atom to be five-coordinate)  

In the final dissociation to give the mixed-halogen species both boron atoms retain their initial coordination number  

Boron-10 isotope substitution studies on Me3N BX3 + BY3 systems, with all possible combinations of X and Y, confirm that the four-coordinate and three-coordinate borons retain their identities (9). Only above 180°C in the gas phase, where dissociation does occur (40), is there loss of isotopic purity (9). 

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Reaction of a donor molecule with a previously equilibrated mixture of free boron trihalides gives an initial adduct mixture corresponding to the equilibrium mixture of the free boron trihalides. This method should be suitable for all mixtures except BF3/BI3 (as noted previously). However, equilibria in the adducts can be quite different from the corresponding equilibria in the free boron trihalides. If halogen redistribution is fast and if the mixed adducts are discriminated against, then this method does not succeed (28). 

The requirement for an initial dissociation step implies that halogen redistribution rates should parallel the ease of dissociation of one of the halogens, and this expectation is confirmed in fluorine - heavier halogen redistributions where the rates change from slow to very fast over the series Cl, Br, I as the B—X bond becomes weaker. Rates should also be related to stabilization of the residual boron trihalide formed on dissociation. The boron trihalide with the greatest number... 

In boron trihalide adducts and tetrahaloborate ions, halogen redistribution equilibria are reasonably close to this ideal random case when chlorine, bromine, and iodine are involved (27, 28, 80, 100, 112), as are equilibria in the uncomplexed heavier boron trihalides (111). 

In the uncomplexed boron trihalides, mixtures of fluorine with chlorine, bromine, and iodine are successively less favored 111), in accord with the symbiotic principle. This principle can rationalize halogen redistribution equilibria in adducts of the mixed boron trihalides as well. Incompatibility of fluorine with the heavier halogens is greatest with the softest donors. In the extreme case of PH3 as... 

Identification of most mixed boron trihalide adducts, and all information on rates and equilibria of halogen redistribution in these systems, depends on correct NMR peak assignments. The reliability of these is assured by a combination of two or more of the following distinctive heteronuclear NMR coupling patterns systematic variation of peak areas as the ratio of two halogens is changed identification and consistent behavior of peaks due to the same species in spectra of two or more of 1H, 19F, UB, and 13C nuclei agreement of observed parameters with values calculated from "pairwise interaction parameters (76,123). Fine points of structure can also be determined by... 

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. 

Mixed boron trihalide adducts of McsP, McaPO, and MesPS are formed by halogen redistribution from the BX3 adducts themselves. For the MesPS... 

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