Under the terms of Citigroup's internal regulations, the bank cannot become involved in a deal where it already holds an advisory interest, meaning that the bank will not launch a hostile takeover bid for the Dutch bank.
However, Citigroup will still be free to bid on any ABN Amro-held assets that Barclays is not interested in acquiring.
Prior to the news of Citigroup taking on an advisory role, it had been perceived as the most likely bank to mount a counterbid following lobbing of chief executive Charles O Prince from high-ranking executives.
It is thought that Citigroup were chosen because of the strength and depth of experience of the company's financial institutions wing, which is currently under the stewardship of Hamid Biglari.
Citigroup will join Credit Suisse, JP Morgan Cazenove, Deutsche Bank and Lazard in the ranks of investment banks acting in an advisory capacity for Barclays.