BACKGROUND Iran has long been trying to acquire the S-300 to bolster its air defenses and protect its nuclear infrastructure. Tehran and Moscow originally signed an $800 million contract for the S-300 in December 2007 after a long and apparently fraught eight-year negotiation. Following the adoption of UN Security Council Resolution 1929 in June 2010 -- which banned the transfer of most arms to Iran, but not SAMs -- Russia announced three months later that it would not fulfill the contract, returning Iran's $166.8 million down payment. In response, Tehran sought punitive damages for the canceled sale, bringing a $4 billion claim against Russia's state-owned arms export company Rosoboronexport to the Court of Conciliation and Arbitration, a Geneva-based arm of the Organization for Security and Co-operation in Europe.
http://www.washingtoninstitute.org/policy-analysis/view/russian-s-300-missiles-to-iran-groundhog-day-or-game-changer