The mechanism of the quinoline transfer hydrogenation by aqueous HCOOH under the action of [Cp*Co(quinNH2)I] + (A*; quinNH2 = 8-aminoquinoline) has been investigated by a combination of experiments and DFT calculations. Variable-temperature (-40 to 20 °C) 1 H NMR in the absence of quinoline substrate shows rapid equilibration between A* and the formate complex [Cp*Co(quinNH2)(O2CH)] + (B*) upon addition of HCOOH/NEt3 in MeOH, yielding ΔH° = 1.49±0.03 kcal mol -1 and ΔS° = 1.92±0.06 cal mol -1 K -1 . This equilibrium mixture slowly converts by decarboxylation and deprotonation to paramagnetic (S = 1) [Cp*Cp(quinNH2)] (C*), indirectly identified by derivatization to [Cp*Co(CN t Bu)2] and further I2 oxidation to [Cp*Co(CN t Bu)2I](I3). The rate law of the [Cp*Co(quinNH2)I] + -catalysed 8-methylquinoline (MQ) transfer hydrogenation with HCOOH in D2O at 80 °C has order one for substrate and catalyst

and order zero for HCOOH, with a rate constant k = (1.52±0.05)⸱10 -2 s -1 mol -1 L. The quinoline (Q) transfer hydrogenation with HCOOH in D2O at 80 °C (k = (2.04±0.05)⸱10 -2 s -1 mol -1 L) selectively yields tetrahydroquinoline doubly D-labelled at the C 3 position ([3,3-D2]-THQ). Under the same conditions, DCOOD in D2O yields [2,3,3,4-D4]-THQ with k = (6.6±0.6)⸱10 -3 s -1 mol -1 L (KIE = kH/kD = 3.1±0.5), while DCOOD in H2O yields [2,4-D2]-THQ. DFT calculations on the Cp model system point to a catalytic cycle with both diamagnetic and paramagnetic intermediates. A key aspect is that transfer of the formate H atom as a hydride to the metal centre, converting [CpCo(quinNH2)(O2CH)] + (B) to [CpCo(quinNH2)H] + (D), is faster than its transfer as a proton to yield [CpCp(quinNH2)] (C). This is at variance with the closely related complex with the 8hydroxyquinoline ligand (ACS Catal. 2021, 11, 11906-11920), underlining the decisive roles of ligand and reaction medium in the selection of the dehydrogenation pathway.