A stability sequence 1 > 2 > 3 > 4 emerges from ab initio calculations for unsubstituted pyrazolylborane. Consideration of the electron correlation interchanges the sequence of 2 and 3. The 11B-NMR chemical shift calculated for 3 (δ = 19.3) lies in a quite different region than those measured for sterically bulky substituted pyrazolylboranes (δ = 63.0−63.3). It therefore appears questionable whether structures of type 3 are realized in solution.
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Schleyer et al. (1990) studied this question.
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