Outlined by a recent New York Times article, the Clinton Foundation received millions of dollars in donations from people affiliated with Uranium One. This company controls over 1/5 of the United States strategic reserves of Uranium, and its sale to the Russian state nuclear company required approval from various officials in the defense and state departments. Both Bill and Hillary Clinton played crucial roles in accepting donations, encouraging the sale, and approving the deal. This sale has created a massive hole in the security of our nation by allowing the Russian government to have so much control over uranium which is a key asset for both America's energy and weapon industry.
More information can be found here:
http://www.nytimes.com/2015/04/24/us/cash-flowed-to-clinton-foundation