Rhodium backbonding

The structure of rhodium(II) carboxylate-derived carbene complexes has been assessed both by quantum mechanical calculations [19,20] and by the study of rhodium(II) carboxylate isonitrile complexes [20,21]. Recent investigations [20] suggest, that also in these highly electrophilic carbene complexes there is a significant n backbonding from rhodium to carbon. 

The situation is now turned to favorable direction in two ways. Eirst, since the inherently electron-rich rhodium(i) is able to donate electrons from the metal center, and thus make vr-backbonding with the electron-deficient multiple bonds much stronger, it initiates the reaction to furnish the cycloaddition products in a highly facile and efficient manner. In addition to the example given below, alkynone 56 and alkynoate 57 also give PKR products in moderate to excellent yields (Equation (28))." " ... 

It has been suggested (162) that there exists only negligible 7r-backbonding in [AuCl(CO>], and a number of displacement reactions have been described (162, 163). Vibrational and NMR spectroscopic studies have been made of this complex (164), and the results have been compared with those for carbonyl complexes of palladium, platinum, rhodium, and iridium. 

In the carbide rhodium clusters, the Rh—C distances of the terminal carbonyl groups are significantly longer than those of the noncarbide derivatives, whereas the corresponding C—0 distances are shorter (see Table II). This trend indicates a decrease in backbonding to the carbonyl... 

Aris et al. (9) examined cyclopentadienyl (propene) irondicarbonyl tri-fluoroacetate and dipropenerhodiumacetylacetonate. In the iron complex the olefinic resonances are shifted upheld by approximately 60 ppm (methylene carbon) and approximately 50 ppm (methine carbon). These shifts indicated, in the authors view, that there was extensive backbonding. For the rhodium complex, the methylene and methine resonances are also shifted upheld upon coordination, but the difference between the upheld shifts (J C(l)-d C(2)) is smaller. 

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