A New Charge Transfer Complex Involving ReH₅(PPh₃)₃

Abstract

A new intermolecular charge transfer (CT) band at 305 nm has been observed in the UV−visible spectrum between ReH5(PPh3)3 and the hydroquinone form of p-chloranil in a mixed CH2Cl2−n-hexanes solution. Formation constants (1600 dm3/mol at 25 °C) have been measured according to a Benesi−Hildebrand treatment and the thermodynamic parameters (−ΔH° = 25.8 kJ/mol; −ΔS° = 25 J/mol; −ΔG° = 18 kJ/mol) have been calculated for this strong CT complexation.

Direct Observation and Structural Characterization of the Encounter Complex in Bimolecular Electron Transfers with Photoactivated Acceptors

“…This finding may seem somewhat anomalous if one considers the significantly different charge-transfer character of the (excited-state) encounter complex relative to the (ground-state) EDA complex as evaluated by the ( b / a ) 2 ratio for the degree of charge transfer in the Mulliken formulation ( vide supra ). For example, the latter can be evaluated directly from the enthalpy and charge-transfer data as ( b / a ) 2 ≃ −Δ H EDA / h ν CT (evaluated in Chart 4 and Table ); and the “degree of charge transfer” is found to be 0.3 in the encounter complex, but only 0.05 in the EDA complex. Thus, the formation enthalpies are rather insensitive to the charge-transfer character of the excited encounter complex relative to the ground-state complex, especially at the limited levels of the degree of charge transfer shown by quinone/aromatic donor−acceptor pairs.…”