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The solution-phase thermolysis of (formazanate)boron dihydrides (LBH2; 2) results in the formation of aminoborane compounds (4) via a series of boron-to-ligand intramolecular hydride transfer reactions. Monitoring the reactions by NMR spectroscopy allowed identification of several intermediates, and a reaction mechanism is proposed. In the case of a ligand with an N-Mes substituent it was possible to characterize an intermediate (7b-i) during this transformation that shows an unexpected cyclohexadiene moiety, which results from hydride transfer to the ortho-position of the mesityl substituent. Two consecutive boron-to-ligand hydride transfers eventually result in reductive N-N bond cleavage to give triazaboroles as the final product. [ABSTRACT FROM AUTHOR]