Rhodium divalent, complexes

These early successes with carbonyl complexes of rhenium encouraged me to undertake systematic research on the carbon monoxide chemistry of the heavy transition metals at our Munich Institute during the period 1939-45, oriented towards purely scientific objectives. The ideas of W. Manchot, whereby in general only dicarbonyl halides of divalent platinum metals should exist, were soon proved inadequate. In addition to the compounds [Ru(CO)2X2] (70), we were able to prepare, especially from osmium, numerous di- and monohalide complexes with two to four molecules of CO per metal atom (29). From rhodium and iridium (28) we obtained the very stable rhodium(I) complexes [Rh(CO)2X]2, as well as the series Ir(CO)2X2, Ir(CO)3X, [Ir(CO)3]j (see Section VII,A). With this work the characterization of carbonyl halides of most of the transition metals, including those of the copper group, was completed. 

Despite the above similarities, many differences between the members of this triad are also to be noted. Reduction of a trivalent compound, which yields a divalent compound in the case of cobalt, rarely does so for the heavier elements where the metal, univalent compounds, or hydrido complexes are the more usual products. Rhodium forms the quite stable, yellow [Rh(H20)6] " ion when hydrous Rh203 is dissolved in mineral acid, and it occurs in the solid state in salts such as the perchlorate, sulfate and alums. [Ir(H20)6] + is less readily obtained but has been shown to occur in solutions of in cone HCIO4. 

The transition-metal catalyzed decomposition of thiirene dioxides has been also investigated primarily via kinetic studies103. Zerovalent platinum and palladium complexes and monovalent iridium and rhodium complexes were found to affect this process, whereas divalent platinum and palladium had no effect. The kinetic data suggested the mechanism in equation 7. 

The more important simple derivatives of cobalt are divalent, the metal only yielding stable trivalent salts in conjunction with other metallic derivatives, as, for example, the cobalti-nitrites and eobalii-cyanides, or in the complex ammino derivatives. Rhodium and iridium function almost exclusively as trivalent metals in their salts. 

It is presumed that hemiporphycene will exhibit a rich metal coordination chemistry. Indeed, preliminary reports have indicated that complexes of divalent magnesium, zinc, nickel, and copper, trivalent iron, cobalt and rhodium, and tetra-valent tin may readily be prepared. Of particular interest in such metalation studies is the fact that metal complexes of hemiporphycene containing an axial substituent (e.g., 3.153 Figure 3.3.3) bear metal-centered chirality because of the dissymmetric nature of the ligand. Unfortunately, further details with regard to this point and other aspects of hemiporphycene coordination chemistry are still lacking at this time. 

The reaction of divalent metals, such as copper, nickel, and so on, with dioxetanes in methanol leads to clean catalytic decomposition into carbonyl fragments/ The reaction rates increase with increasing Lewis acidity of the divalent metal and indicate, therefore, typical electrophilic cleavage of the dioxetane. On the other hand, univalent rhodium and iridium complexes catalyze the decomposition of dioxetanes into carbonyl fragments via oxidative addition. 

In iV-confused porphyrins, this agostic coordination mode has been observed frequently. To date, monomeric complexes of this macrocycle with divalent manganese (35, 26), divalent iron (36, 29), lanthanides (57), rhodium carbonyl (55), and group 12 elements (34), have been structurally characterized that exhibit this peculiar type of metal binding. A generic structure for several of... 

It is well known that chemical shifts for meso protons of the octaethylporphyrin complexes depend upon the oxidation state of the central metal. Divalent metals have a resonance in the region of 9.75-10.08 ppm while for tri- and tetravalent metals this resonance is in the range of 10.13-10.39 and 10.30-10.58 ppm respectively. For the In(OEP)(R) series the indium oxidation state is III and the observed methine proton shifts of 10.17-10.37 ppm correspond to those of a typical trivalent metal (Table 3). The exact position of the In(OEP)(R) methinic protons resonance varies systematically with the electron-donating ability of the axial ligand i.e. the more basic the axial ligand, the higher the field. Similar results are observed for o-bonded complexes of the rhodium series . ... 

The absence of mesomorphism in these compounds was explained on the basis of space-filling requirements. Thus, the intercalation of pyrazine between the binuclear units creates free volume which needs to be filled to obtain a stable, condensed phase when the carboxylates bear only one chain, the interdimeric space is likely filled by the aliphatic chains belonging to a different polymeric chain, giving rise to a crossed structure which prevents the formation of a columnar mesophase. However, as will be seen later, liquid-crystalline behavior was induced in the case of mixed-valence diruthenium(II,III) carboxylate complexes with bulky equatorial Kgands bearing two and three aliphatic chains as with such ligands, it was possible to fill the interdimeric space and thus to induce a thermotropic columnar mesophase. Very recently, the synthesis, characterization, and mesomorphic properties of pyrazine-polymerized divalent rhodium benzoates have also been reported (99). " Most of these compounds exhibit columnar (Colh, Coir, CoIn) and cubic mesophases with melting transition temperatures close to, or even below, room temperature. 

Salts of trivalent rhodium give cherry-red color with tin chloride in the presence of ammonium chloride and potassium iodide. The composition of the red product is unknown but it appears to be a reduction product of rhodium, perhaps a complex of divalent rhodium. 

Particularly interesting is the unusually stable five-coordinate monomeric divalent rhodium complex, [Rh (H)(GO)(PPh3)3], produced by bulk oxidative electrolysis or chemical oxidation of Rh (H)(CO)(PPh3)3 in... 

Silylene-based pincer ligands offer exciting reactivities in terms of transition metal complex formation and their applications in catalytic systems. The pincer complex [SiCSi)Ni(II) can be synthesized by oxidative addition of C—H bond of the corresponding [SiC(H)Si] ligand. [SiCSi]Ni(II) complex has been employed as catalyst for Ni-catalyzed Sonogashira reactions (8). Moreover bis(silylene) pincer complexes of iridium and rhodium reveal strong 5-donating ability of divalent silicon and have demonstrated selectivity in catalytic C—H borylation reactions with arenes (9). 

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