Binuclear cyclopentadienylrhenium hydride chemistry: terminal versus bridging hydride and cyclopentadienyl ligands

Theoretical studies predict the lowest energy structures of the binuclear cyclopentadienylrhenium hydrides Cp₂Re₂H_n (Cp = η⁵–C₅H₅; n = 4, 6, 8) to have a central doubly bridged Re₂(μ–H)₂ unit with terminal η⁵–Cp rings and the remaining hydrides as terminal ligands. However, the lowest energy Cp₂Re₂H₂ structure by more than 12 kcal mol⁻¹ has one terminal η⁵-Cp ring, a bridging η³,η²–Cp ring, and two terminal hydride ligands bonded to the same Re atom. The lowest energy hydride-free Cp₂Re₂ structure is a perpendicular structure with two bridging η³,η²–Cp rings. The previously predicted bent singlet Cp₂Re₂ structure with terminal η⁵-Cp rings and a formal Re–Re sextuple bond lies ∼37 kcal mol⁻¹ above this lowest energy (η³,η²–Cp)₂Re₂ structure. The thermochemistry of the CpReH_n and Cp₂Re₂H_n systems is consistent with the reported synthesis of the permethylated derivatives Cp*ReH₆ and Cp*₂Re₂H₆ (Cp* = η⁵–Me₅C₅) as very stable compounds. Additionally, natural bond orbital analysis, atoms-in-molecules and overlap population density-of-state in AOMIX were applied to present the existence of rhenium–rhenium multiple bonds.