Royal Bank of Scotland is in negotiations to buy the retail bank of Mellon Financial, the Pittsburgh-based financial group. It is understood to be midway through talks on the deal, estimated at $1.8$2.2 billion. The talks are not expected to be concluded for several weeks. The Scottish bank, the UK's second-largest, has been looking since November for acquisitions to add to Citizens, its New England operation, which Bank of Ireland sold out of in 1998.