As far as relaxation effects are concerned, we merely consider the simplest case of relaxation driven by fluctuating local fields experienced by the two protons, ignoring the fluctuations of their mutual dipole-dipole coupling (?). We also assume that the local fields are almost completely correlated, which implies that they efficiently drive the transitions between the triplet states, but not the transitions between the singlet state and triplet states. Hence, in the absence of exchange, singlet-triplet conversion takes infinitely long (in experiments, conversion in the absence of a SABRE catalyst is indeed a very slow process). Precise details and parameters of the model are given in ESI, as well as the method for numerical solution of the set of equations. According to the model outlined above, singlet-triplet conversion in H 2 occurs in the following way. When H 2 binds to the complex, the chemical equivalence is lifted so that coherent transitions between the and the central states become |𝑆〉 |𝑇 0 〉 operative. As a result, the population is distributed between these two states. This gives rise to formation of oH 2 in a nonequilibrium spin state. Subsequently, spin relaxation comes into play and tends to equalize the populations of the three triplet states; eventually, all four states acquire the same population.