Cluster-stabilized cations: synthesis, structures, molecular dynamics and reactivity

Tetrahedral clusters possessing two or three organometallic vertices, such as Co(CO)3 or CpMo(CO)2, provide enormously enhanced stability to neighboring cationic centers. The factors controlling the interactions between the electron-deficient centers and the metals are discussed in terms of their frontier molecular orbitals. Rational syntheses of these systems are now available, and many such metal-stabilized cations have been isolated and characterized not only spectroscopically but also by X-ray crystallography. These species exhibit a variety of fluxional processes which have been elucidated by variable-temperature nuclear magnetic resonance (NMR) measurements and also by a Bürgi-Dunitz analysis of a series of closely related X-ray crystal structures. By preparing clusters containing different organometallic fragments, it is possible to establish a hierarchy of moieties best able to alleviate the electron deficiency at the carbenium ion center. Finally, the growing usage of these metal-stabilized carbocations in organic synthesis, in the elucidation of reaction mechanisms and even in biological chemistry is described.