Observation of Intermediates by Transient Absorption

Perutz reported the time resolved infrared (TRIR) study of CpRh(CO)2 in cyclohexane solution. A species was observed with a lifetime of 15 ms, assigned as CpRh(CO)(c-hexyl)H. If CO is present (1.5 atm), the intermediate decays with a half-life of 1.7 ms. Similar observations were made if CpRh(CO)(C2H4) was used to prepare the reactive intermediate. Laser flash photolysis experiments show the formation of the hydrocarbon activation adduct within 400 ns of the flash, but did not provide evidence for an intermediate prior to its formation [34]. 

The Cp analog of the above system was examined in the gas phase by TRIR, since CpRh(CO)2 is sufficiently volatile to observe intermediates spectroscopically (vapor pressure= 400 mtorr at 20°C). Irradiation in the presence of neopentane (80 mtorr) shows the initial formation of the gaseous species [CpRh(CO)] and its conversion to CpRh(CO)(neopentyl)H. An alkane complex was proposed as an intermediate, but was not detected spectroscopically [38]. 

CO stretch to that in the second intermediate observed in the Tp case provides further evidence for the assignment of the intermediate structures [40]. 

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