The reaction of the heterobimetallic anion, ${(OC)_5CrMn(CO)_5^-M^+ (M^+=Na^+, PPN^+);with;CH_3I$ was proven to be overall 1st order with respect to $[(OC)_5CrMn(CO)_5^-]$ This reaction mechanism may be described in terms of the consecutive reaction pathway in which Cr$(CO)_5$(THF) may be an important intermediate, leading to the corresponding products such as MeMn$(CO)_5;and;ICr(CO)_5^-$, accordingly. The counterion effect on this reaction was also elucidated.