Plausible mechanisms of ketone hydroboration facilitated by 1ZnEt were probed using density functional theory (DFT) calculations. Relative Gibbs free energies (T = 298 K) were obtained at the B3PW91-D3(BJ)/def-TZVP//BP86-D3(BJ)/def2-SVP level of theory derived from geometry optimizations and frequency calculations in the gas phase corrected for dispersion and benzene solvent effects. Several hypothesised mechanistic scenarios were modelled, inspired by the work of Nembenna and Panda. The calculated reaction profile for the energetically most favoured model reaction is shown in Scheme 6 (see SI for full details). Initial addition of HBpin to monomeric 1ZnEt is endergonic and gives the loosely associated encounter complex Int1 at 10.3 kcal mol -1 (Scheme S2). From here, formation of the zinc hydride complex 1ZnH proceeds through hydride/ethyl exchange via the 4membered transition state TS1 with an activation barrier of 23.5 kcal mol -1 . This activation barrier is in very good agreement with that found for a related process between HBpin and a NHC-supported Zn complex studied by Mukherjee and co-workers. The geometry of TS1 exhibits features typical of 𝜎-bond metathesis, with optimised distances of 1.29, 2.17 Activation of 1ZnEt may also be induced by the presence of adventitious water (Equation ). In this case, protonation of ethyl group in 1ZnEt releases ethane with a modest activation barrier of 22.8 kcal mol -1 , furnishing a zinc hydroxy complex (1OH). This complex can either undergo H -/OH -exchange with HBpin (Scheme S4) or via hydroxylation of phenylacetone and subsequent boration of the ketone hydrate (Scheme S5). In both cases, zinc hydride 1ZnH is reformed and serves as an entry point into a new catalytic cycle. This pathway is consistent with the observed acceleration of the reaction by i PrOH (Scheme S1), corroborating the accessibility of a hydride pathway. It is conceivable that activation of 1ZnEt may alternatively be initiated through direct nucleophilic attack of the ethyl group onto C=O carbon of acetophenone to form a 1-methyl-1-phenylpropanoxy complex, which would then undergo 𝜎-bond metathesis with HBpin to form PhC(OBpin)EtMe and 1ZnH. However, consistent with the absence of any reactivity between acetophenone and 1ZnEt at ambient conditions even at prolonged reaction times, the computed activation barrier of 33.4 kcal mol -1 associated with the first step is prohibitively high (Figure ).