We studied co-reactants for tungsten chloride precursors by density functional theory calculation to find the proper reducing agent. Tungsten chlorides, WCl6 and WCl5, are gaining attention for the fluorine-free atomic layer deposition (ALD) of tungsten. We created a W4Cl12 cluster by optimizing the number of tungsten and chlorine atoms in the chlorine-passivated tungsten cluster. We predicted the growth of tungsten carbide by the reaction of trimethylaluminum with the cluster, confirming that the cluster can mimic the chlorine-passivated tungsten surface. Then we simulated the reaction between the W4Cl12 cluster and four co-reactants. Possible reaction pathways between the cluster and the co-reactants were simulated to compare the reaction energies and activation energies. All co-reactants of the present work, atomic hydrogen, H2, SiH4, and B2H6, would act as reducing agents with the reaction energies of −2.07 eV, −0.01 eV, −0.28 eV, and −0.45 eV, respectively. The reducing power was in the order of atomic hydrogen, B2H6, SiH4, and H2 with activation energies of +0.04 eV, +0.18 eV, +1.18 eV, and +2.32 eV, respectively. B2H6 is the most promising gas-phase candidate due to its low activation energy for reduction and high activation energy for boron incorporation.