Non-coordinating anion

Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found as counterions for cationic metal complexes with an unsaturated coordination sphere. These special anions are essential components of homogeneous olefin polymerisation catalysts, where the active catalyst is a coordinatively unsaturated, cationic transition metal complex. For example, they are employed as counterions for the 14 valence electron cations [(C₅H₅)₂ZrR]⁺ (R = methyl or a growing polyethylene chain). Complexes derived from noncoordinating anions have been used to catalyze hydrogenation, hydrosilylation, oligomerization, and the living polymerization of olefins. The popularization of non-coordinating anions has contributed to increased understanding of agostic complexes wherein hydrocarbons and hydrogen serve as ligands. Noncoordinating anions are important components of many superacids, which result from the combination of Bronsted acids and Lewis acids.

Prior to the 1990s, tetrafluoroborate ([BF₄]⁻), hexafluorophosphate ([PF₆]⁻), and perchlorate ([ClO₄]⁻) were considered weakly coordinating anions. These species are now known to bind to strongly electrophilic metal centers. Tetrafluoroborate and hexafluorophosphate anions are coordinating toward highly electrophilic metal ions, such as cations containing Zr(IV) centers, which can abstract fluoride from these anions. Other anions, such as triflates are considered to be low-coordinating with some cations.

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