The electrochemical nitrogen reduction reaction (NRR) is a much sought-after low-energy alternative to Haber–Bosch ammonia synthesis. Single-atom catalysts (SACs) promise to break scaling relations between adsorption energies of key NRR reaction intermediates that severely limit the performance of extended catalysts. Here, we perform a computational screening study of transition metal (TM) SACs supported on vanadium disulfide (VS2) and indeed obtain strongly broken scaling relations. A data-driven analysis by means of outlier detection and subgroup discovery reveals that this breaking is restricted to early TMs, while detailed electronic structure analysis rationalizes it in terms of strong charge transfer to the underlying support. This charge transfer selectively weakens *N and *NH adsorption and leads to promising NRR descriptors for SACs formed of earlier TMs like Ta that would conventionally not be associated with nitrogen reduction.