Carbonyl exchange

At low temperatures, under conditions where carbonyl exchange is su-pressed, the highly reactive (CO)5W=CHPh did not exhibit any carbene exchange of the type in Eq. (4). Rather, the reaction products indicated a direct electrophilic attack on the olefin by the carbene. (The exclusive formation of cyclopropanes will be detailed in Section III.)... 

When the reaction is run in the presence of 13CO and stopped after one half-life, not only is the label incorporation into recovered starting material negligible, as predicted by the mechanism, but the label incorporation into H2Os2(CO)s exceeds that required by the mechanism. This shows that it is undergoing carbonyl exchange under the reaction conditions and suggests that such dinuclear species are much more labile than their mononuclear counterparts. 

In view of the structural similarity between the Os(CO)4 units in Os(CO)4H2 and Os3(CO)i2 noted above, it is interesting to compare the Os(CO)4H2 thermolysis rate, for which we said carbonyl dissociation is rate determining, with the known rate of dissociative carbonyl exchange for Os3(CO)i2 (20). The exchange rate per Os(CO)4 unit extrapolated to 125.8°C is 25 X 10-5 sec-1 the rate of Os(CO)4H2 thermolysis at that temperature is 6 X 10-5 sec-1. The similarity of these numbers is final evidence that carbonyl dissociation from Os(CO)4H2 does occur in the rate-determining step. A vacant coordination site apparently is needed before the actual dinuclear elimination step can occur. 

A. Intermolecular Carbene Exchange and Intramolecular Carbonyl Exchange... 

It was suggested that an increase in the electron density on the cluster due to substitution weakens the metal-metal bonds and thereby facilitates scrambling within the Fe(CO)3 fragment (9). The mechanism of carbonyl exchange between the metals remains unclear. 

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