Hydrocarbon-soluble molybdenum catalyst precursors include a plurality of molybdenum cations that are each bonded with a plurality of organic anions to form an oil soluble molybdenum salt. A portion of the molybdenum atoms are in the 3+ oxidation state such that the plurality of molybdenum atoms has an average oxidation state of less than 4+, e.g., less than about 3.8+, especially less than about 3.5+. The catalyst precursors can form a hydroprocessing molybdenum sulfide catalyst in heavy oil feedstocks. The oil soluble molybdenum salts are manufactured in the presence of a reducing agent, such as hydrogen gas, to obtain the molybdenum in the desired oxidation state. Preferably the reaction is performed with hydrogen or an organic reducing agent and at a temperature such that the molybdenum atoms are reduced to eliminate substantially all molybdenum oxide species.