This review considers cases in which a discrete transition-metal complex is used as a precatalyst for reductive catalysis; it focuses on the problem of determining if the true catalyst is a metal-complex homogeneous catalyst or if it is a soluble or other metal-particle heterogeneous catalyst. The various experiments that have been used to distinguish homogeneous and heterogeneous catalysis are outlined and critiqued. A more general method for making this distinction is then discussed. Next, the circumstances that make heterogeneous catalysis probable, and the telltale signs that a heterogeneous catalyst has formed, are outlined. Finally, catalytic systems requiring further study to determine if they are homogeneous or heterogeneous are listed. The major findings of this review are: (i) the in situ reduction of transition-metal complexes to form soluble-metal-particle heterogeneous catalysts is common; (ii) the formation of such a catalyst is easy to miss because colloidal solutions often appear homogeneous to the naked eye; (iii) a variety of experiments have been used to distinguish homogeneous catalysis from heterogeneous catalysis, but there is no single definitive experiment for making this distinction; (iv) experiments that provide kinetic information are key to the correct identification of the true catalyst; and (v) a more general approach for distinguishing homogeneous catalysis from heterogeneous catalysis has been developed. Additionally, (vi) the conditions under which a heterogeneous catalyst is likely to form include: (a) when easily reduced transition-metal complexes are used as precatalysts; (b) when forcing reaction conditions are employed; (c) when nanocluster stabilizers are present; and (d) when monocyclic arene hydrogenation is observed. Finally, (vii) the telltale signs of heterogeneous catalysis include the formation of dark reaction solutions, metallic precipitates, and the observation of induction periods and sigmoidal kinetics.

A review of the problem of distinguishing true homogeneous catalysis from soluble or other metal-particle heterogeneous catalysis under reducing conditions

Polyhedral skeletal electron pair approach

Amine- and phosphine-borane adducts: new interest in old molecules.

Die Reduktion von C6H5)3PAuCl mit B2H6 in Benzol ergibt Au9,2[P(C6H5)3]2Cl, das mittels Molmassebestimmungen als Au55[P(C6H5)3]12Cl6 charakterisiert wurde. Ein einfaches Modell, beruhend auf einer Anordnung dichtest gepackter Goldatome, fuhrt zu einem Goldcluster, dessen Aufbau mit der ungewohnlichen chemischen Zusammensetzung in guter Ubereinstimmung ist. Das Mosbauer-Spektrum der Verbindung zeigt vier Sorten von Goldatomen: einen metallischen Anteil (Au13-Kern), durch P(C6H5)3-bzw. Cl-Liganden Koordinierte Au-Atome, sowie unkoordi-niertes Oberflachengold. Mit Brom und lod las sich der Komplex zu (C6H5)3PAuCl, (C6H5)3 PAuBr(I) und metallischem Gold abbauen. Die Thermolyse bei 50°C fuhrt quantitativ zu [(C6H5)3P]2AuCl und Gold.

Au55[P(C6H5)3]12CI6 — ein Goldcluster ungewöhnlicher Größe

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Theoretical Studies of Some Transition-Metal-Mediated Reactions of Industrial and Synthetic Importance.

[Rh9P(CO)21]2-. Example of encapsulation of phosphorus by transition-metal-carbonyl clusters

Rhodium-carbonyl cluster chemistry under high pressure of carbon monoxide and hydrogen. 1. Infrared spectroscopic study of homogeneous systems active in the catalytic synthesis of polyalcohols from carbon monoxide and hydrogen

Rhodium carbonyl cluster chemistry under high pressure of carbon monoxide and hydrogen. 3. Synthesis, characterization, and reactivity of HRh(CO)4

[Rh17(S)2(CO)32]3-. 1. An example of encapsulation of chalcogen atoms by transition-metal-carbonyl clusters.

[Rh10P(CO)22]3-. A transition-metal carbonyl cluster with a metal polyhedron based on the bicapped square antiprism as illustrated by the structural study of the benzyltriethylammonium salt

Jose L. Vidal

Papers

[Rh9P(CO)21]2-. Example of encapsulation of phosphorus by transition-metal-carbonyl clusters

Rhodium-carbonyl cluster chemistry under high pressure of carbon monoxide and hydrogen. 1. Infrared spectroscopic study of homogeneous systems active in the catalytic synthesis of polyalcohols from carbon monoxide and hydrogen

Rhodium carbonyl cluster chemistry under high pressure of carbon monoxide and hydrogen. 3. Synthesis, characterization, and reactivity of HRh(CO)4

[Rh17(S)2(CO)32]3-. 1. An example of encapsulation of chalcogen atoms by transition-metal-carbonyl clusters.

[Rh10P(CO)22]3-. A transition-metal carbonyl cluster with a metal polyhedron based on the bicapped square antiprism as illustrated by the structural study of the benzyltriethylammonium salt