The reaction condition space comprised 50 monophosphine ligands, 4 nickel precatalysts, 4 bases, 10 solvents, 3 co-solvents, and 5 temperatures to give a total of 120, 000 combinatorial reaction configurations (see Supplementary Information section 5 for full list). After removing conditions where the reaction temperature exceeded the solvent boiling point, we obtained a reaction condition search space of 88, 000 configurations. Given the large number of categorical variables, which would require 71 one-hot-encoded (OHE) features to describe, we parameterised monophosphine ligands and solvents using quantum mechanical descriptors, which have shown good performance in prior optimisation studies , to provide a more informative representation. We used monophosphine ligand descriptors from Kraken , applying principal component analysis (PCA) to narrow down the 190 ligand descriptors from Kraken to 37 principal components based on a 99% explained variance threshold, labeled ligand_P C1 to ligand_P C37. Solvent descriptors were obtained from Moity et al. with parameterisation using COSMOtherm and represented with 4 DFT-based descriptors labeled Solvent_F 1 to Solvent_F 4 The rest of the categorical variables (cosolvents, bases, and precatalysts) were featurised using OHE, with temperature remaining numerical. The resulting encoded reaction feature space is provided in the data section.