Tensions between the West and Russia over events in Ukraine have cast a shadow over the second round of talks set to begin on Tuesday in Vienna on a permanent nuclear agreement with Iran. Although the talks have no direct connection to Ukraine, their success hinges on solidarity among the so-called P5-plus-one countries — the five permanent members of the United Nations Security Council, which include Russia, plus Germany — in favor of a tough agreement with Iran to drastically scale back its nuclear program. If Russia signals that its cooperation with the West has weakened, that will reduce pressure on Iran to make concessions, said experts knowledgeable about the talks, which began last month with three days of meetings involving senior diplomats from each of the governments involved.