Having confirmed the structure of the bis(crystal violet-borane) adduct 2, UV-Vis analysis was carried out (Figure ). The wavelength of maximum absorbance of crystal violet was 589 nm. No absorption was observed after the addition of BH3 indicating that the adduct formation results in precipitation out of solution. The test conditions (0.5 μM 2, 1.8 mM HBpin, 0.12 mM nucleophile) ensure that the concentration of crystal violet was much lower than the (solvated/ligated) BH3 generated by nucleophilic-mediated decomposition of HBpin or HBcat. The dimethylamine groups of crystal violet are however similar to that of TMEDA, so there is the potential that crystal violet could inhibit any BH3-catalysed hydroboration, and therefore preventing in situ reaction monitoring and 'hidden' catalysis identification. In order to test if an in situ colour indication was possible using crystal violet, without inhibiting reactivity, a BH3-catalysed alkyne hydroboration was carried out in the presence of crystal violet, using an excess of crystal violet (0.22 mmol) relative to Me2S•BH3 (0.4 mmol). A colourless solution was observed after the addition of Me2S•BH3, indicating the formation of the crystal violetborane adduct, however reaction profiling showed the hydroboration of alkyne 1 continued in the presence, and absence, of crystal violet to give the alkenyl boronic ester (83% and 88% respectively). The inhibition of catalysis (BH3-catalysed hydroboration) could therefore be ruled out (see Supporting Information, Figure ). This was further investigated by repeating the reaction using a high loading of the bis-(crystal violet-borane) adduct 2 to catalyse the hydroboration reaction. Product formation was observed, indicating the adduct is sufficiently labile and does not inhibit the reactivity of BH3-catalysed hydroboration (see Supporting Information, Figure ).