[ReH2(CO)2(PMe2Ph)3] +, a dihydride complex with classical and nonclassical tautomeric forms in equilibrium

Article abstract of DOI:10.1021/om00037a044

Reactions of [ReH₃(CO)(PMe₂Ph)₃] (1) with PMe₂Ph or CO in refluxing THF give the monohydride complexes trans-ReH(CO)(PMe₂Ph)₄ (2) and cis,mer-[ReH(CO)₂(PMe₂Ph)₃] (3), respectively. Protonation of 2 with HBF₄·OMe₂ in CH₂Cl₂ at 298 K gives the classical dihydride complex [ReH₂(CO)₂(PMe₂Ph)₃]⁺ (4), while protonation of 3 in CD₂Cl₂ at 193 K results in an equilibrium mixture of the classical dihydride complex [ReH₂(CO)₂(PMe₂Ph)₃]⁺ (5a) and its nonclassical dihydrogen tautomer [Re(η²-H₂)(CO)₂(PMe₂Ph) ₃]⁺ (5b). The η²-HD isotopomer of 5b, formed by protonation of the corresponding monodeuteride complex, shows a ¹J_HD value of 31 Hz in the ¹H NMR spectrum. Variable-temperature ¹H NMR equilibrium studies for the conversion of 5a to 5b produce the thermodynamic parameters ΔH = 1.7 ± 0.4 kcal mol^-1 and ΔS = 8 ± 3 cal mol^-1 K^-1. The lower limit of ΔG^≠ for the conversion of 5b to 5a at 223 K is estimated to be 11.5 ± 0.3 kcal mol^-1.