NDPP secretary general Abu Metha said the coalition is proposed to be named the People's Democratic Alliance.
"The first sitting of MLAs of the alliance partners would be held tomorrow to officially elect Rio as the leader," Metha said.
Rio, a three-time chief minister and a senior leader of the Nationalist Democratic Progressive Party (NDPP), staked claim earlier in the day to form a coalition government in Nagaland, saying he has the support of 32 MLAs.