Kinetics of nucleophilic attack on co-ordinated organic moieties. Part 21. Factors governing the nucleophilicity of phosphorus nucleophiles towards [Fe(CO)3(1–5-η-C6H7)]+

Abstract

Synthetic and kinetic studies are reported for the addition of a wide range of tertiary phosphines and phosphites to the dienyl ring of [Fe(CO)₃(1–5-η-C₆H₇)]⁺(1). All the reactions obey the expression k_obs.=k₁[PR₃], except for the reversible additions of P(CH₂CH₂CN)₃ and P(C₆H₄Me-2)₃ which follow the two-term equation, k_obs.=k₁[PR₃]+k_–1. A good correlation (r= 0.98) is found between log k₁ and the Tolman Σχ values of the phosphorus nucleophiles. The Brönsted slope of 0.5 derived from a linear plot of log k₁vs.pK_a for triarylphosphines demonstrates the importance of basicity in controlling nucleophilicity towards (1). Nucleophile polarisability is shown to play a minor role. Steric factors, quantified by negative deviations from the above plots by P(C₆H₄Me-2)₃ and P(C₆H₁₁)₃ are shown to be much more important than in related reactions with ethyl iodide. The Hammett slope ρ of –1.32 observed for the additions of triarylphosphines to (1) suggests moderate, but far from complete, phosphorus–carbon bond formation in the transition states. A similar conclusion is drawn from the novel anchimeric assistance observed in the unexpectedly rapid reaction with P(C₆H₄OMe-2)₃(k_2–MeO/K_H= 91). Finally, comparison with other results provides an electrophilicity order for an extended series of organometallic cations covering a reactivity range of ca. 10⁶ between [Mn(CO)₃(1–6-η-C₇H₈)]⁺ and [Co(η-C₅H₅)(1–5-η-C₇H₉)]⁺.