Intramolecular Dynamics of Five‐coordinate iron Carbonyl Complexes with olefinic ligands as studied by variable‐pressure 1 H‐NMR spectroscopy

Variable‐pressure 1 H‐NMR Spectroscopy has been used to study the fluxionality of some five‐coordinated Fe complexes in solution. For [Fe(CO) 2 1,3‐cyclooctadiene (PPh 3 )], the CO site exchange is known (by analogy with [Fe(CO) 3 (1,3‐cyclooctadiene)]) to be a non‐dissociative process, and an activation volume of ca. 0 cm 3 .mol −1 was indeed obtained. However, for [Fe(CO 2 ){2,3‐η: O ‐σ‐(7,7‐dimethoxybicyclo[2.2.1]hept‐2‐ene)}(PPh 3 )], the activation volume of +5 cm 3 mol −1 suggests that an unprecedented dissociation process is responsible for the CO site exchange. The molecular structure of [Fe(CO) 2 (1,3‐cyclooctadiene)(PPh 3 )] was ascertained by single‐crystal X‐ray diffractometry. The crystals are triclinic, space group P 1 , a = 9.606(3), b = 16.795(2), c = 7.743(8) Å, α = 97.83(4), β = 109.63(4), γ = 83.37(2)°. The structure determination has shown that the complex possesses a tetragonal pyramidal coordination, with the endocyclic CC bond and PPh 3 occupying basal sites.