The -bond metathesis phase proceeds through addition of the (H-BBN)2 dimer to 10 to form INT-1Zn-B at +20.4 kcal/mol. This involves the C position of the NacNac ligand acting as a Lewis base towards one H-BBN unit, with the other H-BBN unit interacting with the Zn-thienyl moiety. INT-1Zn-B therefore contains two B-HZn interactions, with that bridging the Zn- INT-2Zn-B in which the Zn-H 1 B 1 moiety approaches linearity (165°) and from which dissociation of the thienyl-BBN product proceeds via TS2Zn-B with concomitant contraction of the B 2 H 2 Zn distance to 1.83 Å. Thus, the non-innocence of the NacNac ligand not only assists the cleavage of the (H-BBN)2 dimer by C-B bond formation, but the resultant C-bound hydroborane provides flexible ligation of the zinc centre during the metathesis phase. This is also reflected in the variation of the Zn-NacNac metallacycle folding angle, , as previously discussed (inset Figure (b)) and is also seen in ). However, dissociation of 0.5 equiv. of (H-BBN)2 from 11 to form NacNacZnH, 1, is endergonic by only 7.4 kcal/mol. Complex 1 can then react with [(DMT)H] + via the mechanism described in our previous work which proceeds by protonation of the C position of NacNac. Note, the highest-lying transition state for this process lies at +26.9 kcal/mol and so is competitive with TS3Zn-B at +27.6 kcal/mol in the -bond metathesis phase. However, we disfavor this protonolysis step being rate limiting as that scenario would result in an inverse dependence on [(H-BBN)]2, not the first order dependence observed. Finally, as with the bond metathesis phase, we highlight that NacNac ligand non-innocence again appears essential for accessing sufficiently low barrier mechanisms for the protonolysis phase.