MILAN, Dec 16 (Reuters) - Italy"s Nexi (NEXII.MI) put the final word to its tie up with domestic rival SIA on Thursday by signing the merger deed to create one of Europe"s largest payment groups. The merger with SIA, which follows Nexi"s deal with Nordic payment group Nets, will be effective as of Jan. 1, 2022, Nexi said in a statement. Nexi Chief Executive Paolo Bertoluzzo has been confirmed as CEO of the new group until approval of the 2024 financial statement, Nexi said.