Matthew Kroenig argues that Russia and China won't stop Iran from acquiring the bomb:
[An Iranian bomb] won’t disadvantage China or Russia. In fact, it might even help them. Neither country has hidden its desire to hem in America’s unilateral ability to project power, and a nuclear-armed Iran would certainly mean a more constrained U.S. military in the Middle East.
Indeed, at times during the 1980s and 1990s, Beijing and Moscow aided Tehran with important aspects of its nuclear program. While we don’t have detailed information on the motives behind the assistance, we do know that governments don’t export sensitive nuclear technologies for economic reasons alone. Rather, as I show in my forthcoming book, they generally do so in an attempt to hinder their enemies. For example, France helped Israel acquire the bomb in the late 1950s and early 1960s in order to balance against Nasser’s Egypt, and China provided nuclear aid to Pakistan in the 1980s to impose strategic costs on its longtime rival India.It is likely that China and Russia’s nuclear assistance to Iran waspartly intended as a counterweight to American power in the Middle East. Although these countries no longer actively aid Iran’s nuclear program, they may still secretly welcome its development.