Rhodium 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... 

Ans. In the BASF process more Fischer-Tropsch by-products are formed. In Rh -based catalysis rhodium is maintained in the lower oxidation state, ensuring that 4.1 concentration is higher than 4.12 and the acetic acidforming cycle dominates. 

A totally new field of rhodium chemistry was opened up by the discovery of the remarkable catalytic properties of [RhCl(PPh3)3] by Wilkinson s school.9,10 This important complex has stimulated a rapid expansion in the chemistry of the lower oxidation state complexes. Much of this interest has, inevitably, spilled over into the organometalic chemistry of the element and is thus outside the scope of this chapter. 

Rhodium does not readily form anionic complexes in its lower oxidation states, probably because rc-acid ligands are usually required to disperse the electron density from the metal in neutral or even cationic complexes. It is not surprising, therefore, that the few examples of rhodium(I) anionic complexes contain powerful rc-acid ligands. 

On the other hand, rhodium(III) complexes containing r-bonded ligands are often pentacoordinate and undergo very rapid reductive elimination reactions. It is their ability to undergo this reaction that makes possible much of the important catalytic activity exhibited by lower oxidation state rhodium complexes. 

Saturation of a carbohydrate double bond is almost always carried out by catalytic hydrogenation over a noble metal. The reaction takes place at the surface of the metal catalyst that absorbs both hydrogen and the organic molecule. The metal is often deposited onto a support, typically charcoal. Palladium is by far the most commonly used metal for catalytic hydrogenation of olefins. In special cases, more active (and more expensive) platinum and rhodium catalysts can also be used [154]. All these noble metal catalysts are deactivated by sulfur, except when sulfur is in the highest oxidation state (sulfuric and sulfonic acids/esters). The lower oxidation state sulfur compounds are almost always catalytic poisons for the metal catalyst and even minute traces may inhibit the hydrogenation very strongly [154]. Sometimes Raney nickel can... 

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. 

A simplified reaction scheme is shown in Fig. 26.5 Again, the ability of rhodium to change its coordination number and oxidation state is crucial, and this catalyst has the great advantage over the conventional cobalt carbonyl catalyst that it operates efficiently at much lower temperatures and pressures and produces straight-chain as opposed to branched-chain products. 

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. 

The catalytic cycle involves the same fundamental reaction steps as the rhodium system oxidative addition of Mel to Ir(I), followed by migratory CO insertion to form an Ir(III) acetyl complex, from which acetic acid is derived. However, there are significant differences in reactivity between analogous rhodium and iridium complexes which are important for the overall catalytic activity. In situ spectroscopy indicates that the dominant active iridium species present under catalytic conditions is the anionic Ir(III) methyl complex, [IrMe(CO)2l3] , by contrast to the rhodium system where the dominant complex is [Rh(CO)2l2] - PrMe(CO)2l3] and an inactive form of the catalyst, [Ir(CO)2l4] represent the resting states of the iridium catalyst in the anionic cycles for carbonylation and the WGSR respectively. At lower concentrations of water and iodide, [Ir(CO)3l] and [Ir(CO)3l3] are present due to the operation of related neutral cycles . 

The number and variety of compormds in oxidation states of four or greater is dramatically lower than for rhodium(III). Essentially only oxo and halo species are formed, and none of these are cationic. 

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. 

The number of complexes of this type is considerably higher than for inclusion-and carbonyl complexes, because the components of the complexes can be varied much more Substituted cyclophanes from [22]- up to [26]cyclophanes, as well as indenophanes are used as ligands for metal fragments, which may consist of chromium, iron, ruthenium, rhodium, iridium or cobalt as metal units, and Cp-, Cp - and C R -units as stabilizing co-ligands. From Fig. 21 the importance of the oxidation state and position in the periodic table of the individual metals can be seen [48a] cobalt-III forms monocomplexes only, whereas the lower sited iridium-IIl also forms the biscomplexes for cobalt. 

The lanthanide contraction, however, has also effects for the rest of the transition metals in the lower part of the periodic system. The lanthanide contraction is of sufficient magnitude to cause the elements which follow in the third transition series to have sizes very similar to those of the second row of transition elements. Due to this, for instance hafnium (Hf ) has a 4" -ionic radius similar to that of zirconium, leading to similar behavior of these elements. Likewise, the elements Nb and Ta and the elements Mo and W have nearly identical sizes. Ruthenium, rhodium and palladium have similar sizes to osmium iridium and platinum. They also have similar chemical properties and they are difficult to separate. The effect of the lanthanide contraction is noticeable up to platinum (Z = 78), after which it no longer noticeable due to the so-called Inert Pair Effect (Encyclopedia Britannica 2015). The inert pair effect describes the preference of post-transition metals to form ions whose oxidation state is 2 less than the group valence. 

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