An arbitration agreement or arbitral award arising out of a legal relationship, whether contractual or not, which is considered as commercial, including a transaction, contract, or agreement described in section 2 of this title, falls under the Convention. An agreement or award arising out of such a relationship which is entirely between citizens of the United States shall be deemed not to fall under the Convention unless that relationship involves property located abroad, envisages performance or enforcement abroad, or has some other reasonable relation with one or more foreign states. For the purpose of this section a corporation is a citizen of the United States if it is incorporated or has its principal place of business in the United States.