Opportunistic or a testament to its strong ties, balance sheet and global M&A profile?

Of the dozen or so banks involved in miner Rio Tinto's year-long takeover saga, only JPMorgan has served three different roles to two separate clients.

JPMorgan showed its muscle as it switched sides during the Rio deal, but also raised some eyebrows.

The bank's swift move from advising Chinese client Chinalco on its $19.5 billion offer for Rio assets and a doubling of its stake in the miner to helping Rio underwrite a bumper rights issue has prompted rival bankers to query JPMorgan's loyalties.

The New York bank, Rio's corporate broker in London via affiliate JPMorgan Cazenove, advised the Anglo-Australian miner in fending off a hostile bid by rival BHP Billiton that collapsed late last year.

It then gained a last-minute advisory role for Aluminum Corp of China (Chinalco), helping the metals company as it sought to buy into Rio Tinto.

Rio eventually ditched that plan, opting instead for a $15.2 billion rights issue and an iron ore joint venture with BHP. One of the underwriters mandated for the rights offer: JPMorgan.

JPMorgan's flip-flopping between clients breaks no rules, and some rival investment bankers say its manoeuvring is a credit to its business relationships, financial muscle and top status in the global M&A advisory league tables.

Others say it may put itself in an awkward position, even though the bank's latest mandate with Rio is for securities underwriting and not M&A advice.

"If both sides are comfortable, fine," said a rival investment banker involved in the Rio-Chinalco deal. "It's just strange, though. You're effectively undermining Chinalco's ability."

Both JPMorgan and Chinalco declined to comment for this article. The sources declined to be named because they were not authorised to speak publicly on the matter.