Unusual Structure and Reactivity of the Photogenerated Intermediate Cp*Cr(μ-CO)₃CrCp* (Cp* = η⁵-C₅Me₅)

Abstract

Employing time-resolved IR spectroscopy, we have demonstrated that, following photolysis of [Cp*Cr(CO) 2 ] 2 ( 1 ) in n -heptane, the only initial photoproduct is the triply bridged CO-loss intermediate Cp*Cr(μ-CO) 3 CrCp* ( 4 ). Two bridging ν(CO) bands are observed, resulting from the splitting of the e‘ mode of 4, which leads to the conclusion that the CO bridging groups are asymmetric. The rate of the back reaction of 4 with CO has been measured ( k 2 = 5.2 (±0.5) × 10 7 M -1 s -1 ), and this is 1000 times faster than the rate of the analogous back reaction of CO with CpFe(μ-CO) 3 FeCp. Moreover, unlike many CO-loss intermediates, there is no reaction of 4 with PPh 3 . The rate of reaction of 4 with THF, to produce Cp* 2 Cr 2 (CO) 3 (THF) ( 6 ), has been estimated ( k 2 = 1.6 (±0.2) × 10 8 M -1 s -1 ).