Mechanisms of carbon monoxide replacement in metal carbonyls

The late Professor Walter Hieber, the ‘father’ of metal carbonyl chemistry, at the 3rd International Conference on Coordination Chemistry (ICCC) held in Amsterdam in May, 1955, gave a plenary lecture summarizing the elegant research done in his laboratory on the reactions and syntheses of metal carbonyls. I was fortunately present at his lecture, after which I congratulated him and his students for all of the fine work they had done and I asked if they had any knowledge as to how some of these reactions take place. With the aid of an interpreter, Professor Hieber kindly responded “we have been interested in real chemistry (reactions and syntheses) not the philosophy of chemistry (bonding and mechanisms)“. His talk and his comment were largely responsible for my becoming interested in doing some work on the kinetics and mechanisms of metal carbonyls. Very little research of this type had been reported, and in the late fifties there was not much interest in the chemistry of metal carbonyls. This meant that almost anything we did would generate new publishable results, and also the field was not crowded with large numbers of investigators all rushing to do the same or similar experiments. One would think that graduate students should want to work on such problems which are almost certain to lead to publications and to acceptable doctoral theses. Yet, at first, students were reluctant to choose a thesis problem dealing with metal carbonyls because all of the other people in my research group were working on Werner complexes in aqueous solution. Students felt more comfortable working with Werner complexes than with toxic metal carbonyls. Wojcicki was the first brave student willing to work with metal carbonyls, and largely because of his excellent work it later developed that students often preferred to work with these systems. should stick to the simple binary metal carbonyls and to their ‘uncomplicated’ carbon monoxide exchange. We decided to work with only the first row transition metals and to ask and seek answers to questions such as (1) the effect of coordination number on the rates and mechanism of reaction, and (2) whether bridging carbonyls and terminal carbonyls react at different rates. We were in for several surprises, because little had been done on the mechanisms of substitution reactions of metal carbonyls and we had little to base any ‘educated’ guesses on. We and others had, however, done considerable work of this type on the substitution reactions of the classical metal complexes [l] . Most of that work was on the six-coordinate cobalt(II1) complexes and on the fourcoordinate platinum(H) complexes. The general picture that had emerged from those studies was that octahedral substitutions take place by dissociative Sol (or Id) [2] processes, whereas square planar substitutions are associative SN2 or (13 reactions. Because of this, we were surprised to find that the four-coordinate Ni(C0)4 reacts by means of a dissociative process [3]. Our initial experiments on the rates of carbon monoxide exchange were done by circulating r4C0 through a solution of Ni(C0)4 and monitoring the change or radioactivity of the CO in the gas phase. Because of experimental difficulties the experiments had to be repeated later by Day [4] using Cl80 and an infrared technique. He also carried out substitution reactions using triphenyl phosphine. The data collected fit the rate law (1)