Rhodium with lower oxidation states

The most common oxidation state of rhodium in aqueous solution is low-spin 4d > rhodium(III). The lower oxidation state rhodium(I) has been extensively studied in organic solvents. By contrast, in aqueous solutions few studies of the lower oxidation states of rhodium have been made Gray with Mann, Sigal and... 

Therefore 4d and 5d electron metals interact with ligands in a more effective manner and thus form more covalent compounds. Because of valence orbital energy and orbital sizes, compounds of these elements in their lower oxidation states, particularly organometallic ones, are more stable than analogous complexes of M electron metals. The increased stability of olefin and acetylene compounds with increasing atomic number in a given group may serve as an example. Olefin complexes of cobalt are few and very unstable, while rhodium and iridium olefin compounds are quite common and usually air-stable. 

The mechanism for the reaction is believed to be as shown in Eq. 15.170 (start with CH3OH, lower right, and end with CHjCOOH, lower left).180 The reaction can be initiated with any rhodium salt, e.g., RhCl3, and a source of iodine, the two combining with CO to produce the active catalyst, IRItfCO y. The methyl iodide arises from the reaction of methanol and hydrogen iodide. Note that the catalytic loop involves oxidative addition, insertion, and reductive elimination, with a net production of acetic acid from the insertion of carbon monoxide into methanol. The rhodium shuttles between the +1 and +3 oxidation states. The cataylst is so efficient that the reaction will proceed at atmospheric pressure, although in practice the system is... 

The volume of space occupied, per fluorine atom, has often been quoted as about 18 A3. For the transition metal fluorides this is approximately so, although there is considerable variation, and the values are more often lower. For a particular metal, the variation in volume occupied with change in oxidation state is not simple. Thus for vanadium the volumes are VF2, 19.5 VF j, 17.2 VF4, 16.1 VF5, 16.1 A3, whereas for rhodium the values are RhF j, 15.5 RhF4, 15.5 RhF5, 16.9 RhFfi, 16.7 A3. 

First, a direct extension of this thinking leads to the conclusion that high oxidation states are most likely to be achieved in an anion, where, as a consequence of the electron-rich environment, the electronegativity of the high oxidation state is lower than in neutral or cationic species. This thought led to the discovery of the room temperature oxidation of gold and the platinum metals (all except rhodium react) using F2 in aHF made basic with alkali fluorides, described in Ref. 112. 

More often in organometallic chemistry, the catalytic reaction occurs by a mechanism that is completely different from the mechanism of the uncatalyzed process. In this case, the reaction typically occurs by more steps, but the activation energy of each of the individual steps is lower than the activation energy of the imcatalyzed process. The overall barrier is then lower than that of the uncatalyzed reaction. A comparison of the uncatalyzed and catalyzed hydroboration of alkenes with a dialkoxyborane (ROl BH, such as cat-echolborane (see Chapter 16), illustrates this scenario. Qualitative reaction coordinates for tihe uncatalyzed and rhodium-catalyzed process are shown in Figure 14.4. In the absence of a catalyst, the B-H bond adds across the alkene through a concerted four-center transition state, albeit at elevated temperatures in neat alkene. hi contrast, late transition metal-catalyzed hydroborations first cleave the B-H bond by oxidative addition. Coordination... 

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