Actually, cobalt is routinely found as Co(II) and Co(III) salts, the interesting complexes, as reducing agents, are Co(I) complexes. but in general, when we want to synthesize AgNPs or other metal nanoparticles, the routine reducing agents are used (e.g NaBH4, Sodium citrate and etc...). anyway, please find the attached papers I hope you will find it useful and practical.