Nikkei at 3-yr closing low, hit by global credit fears

*Nikkei falls 2.2 percent at three-year closing low

*Financial shares hit by global credit fears

*Sony plunges to hit five-year low after Goldman downgrade (Adds stocks and comments)

By Aiko Hayashi

TOKYO, Sept 18 (Reuters) - The Nikkei average slid 2.2 percent to a three-year closing low on Thursday, with financial shares tumbling as the growing crisis on Wall Street sent major U.S. banks scrambling for merger partners.

Sony Corp plunged to ...