AbstractThree new η2‐acyl complexes, TMo(CO)2(η2‐COR) (T = Tp', Tp'; R= Me, Bun; Tp' = hydridotris(3,5‐dimethyl‐pyrazol‐1‐yl)borate; Tp' = hydridotris(3,4,5‐trimethylpyrazol‐l‐yl)borate) prepared from the corresponding alkyl iodides, RI, and the anions, TMo(CO)−3. The preparation of Et4N+Tp Mo(CO)3−is also described. Using solution IR spectra to monitor the reactions between RI and TMo(CO)3−, it was found that Tp'Mo(CO)2(η2‐COMe) was formed more readily than Tp'Mo(CO)2(η2‐COMe) which was obtained more easily than Tp'Mo(CO)2(η2‐COBun). This finding suggests that the mechanism is probably an ionic substitution rather than a radical mechanism. The different times required for the complete conversion from the anions to TMo(CO)2(η2‐COR) is rationalized in terms of the electro