Determination of phosphine basicity and study of nucleophilic addition to [pi]-hydrocarbon ligands
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It has been recognized for some time that many low valent transition metal complexes can act as Lewis bases. Unlike organic bases, metal complexes have not been systematically investigated to determine what factors might influence their electron donating ability;As part of an effort to study metal complex basicity, titration calorimetry has been employed to determine the heats of protonation ([delta]H[subscript] HP's) of several phosphines commonly encountered in metal complexes. The phosphines are protonated with CF[subscript]3SO[subscript]3H in 1,2-dichloroethane solvent. The [delta]H[subscript] HP values are direct and accurate measures of phosphine basicity, and the method can be extended to basicity determination over a wide range of base strength;The ability to activate [pi]-hydrocarbon ligands toward nucleophilic attack is another important characteristic of transition metals. This feature has been extensively exploited in both large scale industrial processes and laboratory scale syntheses. Reactivity of the [pi]-hydrocarbon ligand in transition metal complexes can vary widely with changes in the nature of the metal or ancillary ligands; attempts to correlate the observed reactivity with experimentally determined parameters have previously been unsuccessful;A new method for predicting the reactivity of coordinated ethylene and benzene toward nucleophilic addition is described in this work. This scheme employs metal carbonyl C-O force constants to gauge the activating ability of a metal-ligands fragment to which ethylene or benzene is bound. The approach has proven quite capable of correlating the reactivity of numerous complexes with nucleophiles;In a related study, the activating ability of a metal carbonyl moiety was used to promote addition of a metal-based nucleophile to coordinated cycloheptatriene. The anion Re(CO)[subscript]5[superscript]- reacts with ([eta]-C[subscript]7H[subscript]8)Mn(CO)[subscript]3[superscript]+ to give the bimetallic complex, (1-5-[eta]-6-exo-Re(CO)[subscript]5-C[subscript]7H[subscript]8) Mn(CO)[subscript]3. This is the first example of metal carbonyl anion addition to a coordinated cyclic [pi]-hydrocarbon.