 The loss of CO is rate limiting making the investigation of this reaction mechanism difficult.The alkyne then coordinates to the metal center which has been calculated as a low-energy barrier process with a ΔE‡ of 2.0-4.0 kcal/mol (for terminal alkynes at B3LYP/SVP).The resulting alkyne metal coordinated complex immediately rearranges to intermediate 4 with activation barrier of 3-9 kcal/mol (for terminal acetylenes at B3LYP/SVP).