HARTFORD — The Phoenix Companies. Inc. is being sold to Nassau Reinsurance Group Holdings L.P. for $217.2 million, the companies announced early Tuesday. That's a huge premium for shareholders, whose stocks were worth $13.03 at closing Monday, and who will be offered $37.50 a share. The stock was trading at about $32.75 after the announcement. The stock had plunged from $56.07 on March 30 to $17.85 on May 14, the day of its annual meeting. Bad news on financial restatements,