Citigroup has bought Deutsche Bank’s metals and energy-trading books, according to people familiar with the situation, as the US banking giant expands its investment in the commodities business.

Citi bought Deutsche's metals, power and oil-trading books in a series of separate transactions over the past few months, with the purchase of the oil book completed as recently as a few weeks ago, according to one of the people familiar with the situation. It was unclear what Citi paid for the portfolios.