Whenever there are moneys in the city treasury which will not be required to be used for a period of six months or more, the Director of Finance shall submit to the Mayor and to the Director of Law, a statement of moneys in the Treasury or in the process of collection, and a schedule showing the probable requirements of money for the use of the city for such period of not less than six months as the Mayor may direct, together with a recommendation as to whether any moneys in the Treasury shall be invested in obligations as provided by Ohio R.C. 731.56. The Mayor, Director of Law and the Director of Finance may thereupon order such investments of moneys in the Treasury in such obligation, at not more than the current market value, as they may deem advisable in the interest of the city. Other than purchases of Federal Treasury securities, no investment shall be made except in obligations which have been passed upon and approved as to validity by a reputable firm of bond attorneys.