Kinetics of nucleophilic attack on coordinated organic moieties: XXVII. Kinetics and mechanism of addition of tertiary phosphines and phosphites to [Co(C 5H 5)(η 5-C 8H 11)] + and related cobalt cations

Abstract

Kinetic data are reported for the addition of a range of tertiary phosphines and phosphites to the dienyl cation [Co(C 5H 5)(η 5-C 8H 11)] (I) (eq. 1). The rate law depends on the nature of the nucleophile. For example, the reactions with PBu n 3 and P(OBu n) 3 obey the expression k obs = k 1 [PR 3], while the two-term equation k obs = k 1 [PR 3] + k −1 is followed for the reversible addition of PPh 3. The low enthalpies of activation (Δ H # 1 15–36 kJ mol −1) are consistent with bimolecular addition of PR 3 to the C 8H 11 ligand, as are the large negative entropies of activation (Δ S # 1 −91 to −124 J K −1 mol −1). The basicity of the phosphorus nucleophiles is shown to be important (Bronsted slope, α, ca. 0.5) in determining the nucleophilicity towards cation I. An excellent linear free energy relationship is found between reactions 1 and the related additions of phosphines and phosphites to the cation [Fe(CO) 3(1–5-η-C 6H 7)] +, suggesting similar transition states for each process. ▪ The analogous additions of PBu n 3 to the cations [CoC 5H 5(1–5-η-C 6H 7)] + (IIIa) and[Co(C 5H 5)(1–5-η-C 7H 9)] + (IIIb) are very much slower. The relative electrophilicities of the cations I ⪢ IIIa ⪢ IIIb (rel. rates 4 × 10 4/2.5 × 10 2/1) are found to be enthalpy controlled (Δ H # 1 15, 31 and 38 kJ mol −1, respectively).