Reactions of B 2 ( o ‐tolyl) 4 with Boranes: Assembly of the Pentaborane(9), HB[B( o ‐tolyl)(μ‐H)] 4

Reactions of the diborane(4) B 2 ( o ‐tolyl) 4 and monohydridoboranes are shown to give B( o ‐tolyl) 3 and (o‐tolyl)BR 2 (R 2 =(C 8 H 14 ) 3 , cat 4 , pin 5 , (C 6 F 5 ) 2 6 ) as the major products. The corresponding reaction with BH 3 ‐sources gives complex mixtures, resulting from hydride/aryl exchange, dimerization and borane elimination. This led to the isolation of the first tetra‐substituted pentaborane(9) HB[B( o ‐tolyl)(μ‐H)] 4 8 . The reaction pathways are probed experimentally and by computations.