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.