Reduction of several metal carbonyl dimers including Mn2](CO)10, [C5H5Fe(C0)2]2, Co2(CO)8, and [C5H5M(CO)3]2 (M = Cr, Mo and W) by sodium-potassium alloy (NaK) in tetrahydrofuran at room temperature provides a rapid and clean method for the production of the corresponding metal carbonyl anions in high yield. Isolation and characterization of [n-Bu4N] [Fe(CO)2C5H5] from the iron dimer reduction is described. Reductions of other carbonyls including M(CO)6 (M = Cr, Mo and W) and Re2(CO)10 proceed more slowly than previously established methods and provide principally M2(CO)102- and Re(CO)55-. Methods for the preparation of Re(CO)5- are critically considered. The reaction of NaK with [C5H5NiCO]2 is discussed in relation to previously reported results. Infrared solution spectra of a number of carbonyl anions in THF, obtained in a special infrared solution cell, are reported.