Hydrocarbon-soluble molybdenum catalyst precursors include a plurality of molybdenum cations and a plurality of carboxylate anions having at least 8 carbon atoms. The carboxylate anions are alicyclic, aromatic, or branched, unsaturated and aliphatic, and can derived from carboxylic acids selected from 3-cyclopentylpropionic acid, cyclohexanebutyric acid, biphenyl-2-carboxylic acid, 4-heptylbenzoic acid, 5-phenylvaleric acid, geranic acid, 10-undecenoic acid, dodecanoic acid, and combinations thereof. The molybdenum salts have decomposition temperatures higher than 210° C. The catalyst precursors can form a hydroprocessing molybdenum sulfide catalyst in heavy oil feedstocks. Also disclosed are methods for making catalyst precursors and hydrocracking heavy oil using active catalysts.