Square planar rhodium

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

For tetranuclear cluster complexes, three stmcture types are observed tetrahedral open tetrahedral (butterfly) or square planar, for typical total valence electron counts of 60, 62, and 64, respectively. The earliest tetracarbonyl cluster complexes known were Co4(CO)22, and the rhodium and iridium analogues. The... 

Reaction of [Rh(/z-Cl)(CO)2]2 with sodium pyrazolate leads to 206 (85CJC699). The Rh2N2Cl ring has the envelope conformation. The rhodium atom has distorted square-planar coordination. The molecules in the crystalline lattice form onedimensional stacking units with alternating rhodium atoms in the binuclear units, intermolecularly interacting in a zigzag chain. 

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

The double doublet corresponds to Pfi, with splitting owing to phosphorus A (cis) (/(P-P) 25 Hz) and rhodium (/(Rh-P) 172 Hz). The fluxional behaviour is consistent with a rapidly rearranging (at room temperature) square planar structure rather than a tetrahedral one (Figure 2.11). 

Rh(Ph2P(CH2)2PPh2)2 CIO4 has essentially square planar coordination of rhodium (Rh-P 2.289-2.313 A) [54],... 

Trimesityl rhodium is reduced by PMe2Ph to give a square planar rhodium(I) aryl... 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

Oxidative addition of RX to square-planar Ir(CO)L2X or to CpIr(CO)L (L = PR3 or ASR3) invariably affords RIr(CO)L2X2 (71, 85, 86) and CpIr(CO)LR X (106, 198), respectively. Unlike some of their rhodium analogs, these complexes do not rearrange to the acyls. 

The hydrosilylation of acetophenone by diphenylsilane in CH2CI2 at rt was used as a test reaction to compare the selectivity obtained with the carbene ligands (Scheme 36). The reactions were performed in the presence of a sUght excess of AgBp4 (1.2% mol). In these conditions, the N-mesityl-substituted catalyst 57c (1% mol) gave the highest selectivity (65% ee). The in situ formation of square-planar cationic rhodium species 58 as active catalysts appears to be crucial since the same reaction performed without silver salt gave both poor yield (53%) and enantioselectivity (13%). 

S. J. Pool and K.H. Shaughnessy. Effects of ionic liquids on oxidative addition to square planar iridium and rhodium complexes. Abstracts of Papers, 231st ACS National Meeting, Atlanta, GA, USA, March 26-30,2006 (2006). 

Remarkably, Claver et al. showed that in a square planar rhodium carbonyl chloride complex, two bulky phosphite ligands (65) were able to coordinate in a trans orientation.214 Diphosphite ligands having a high selectivity for linear aldehyde were introduced by Bryant and co-workers. Typical examples are (67)-(70).215,216... 

The mechanistic basis of iridium-complex-catalyzed enantioselective hydrogenation is less secure than in the rhodium case. It is well known that square-planar iridium complexes exhibit a stronger affinity for dihydrogen than their rhodium counterparts. In earlier studies, Crabtree et al. investigated the addition of H2 to their complex and observed two stereoisomeric intermediate dihydrides in the hydrogenation of the coordinated cycloocta-1,5-diene. The observations were in contrast to the course of H2 addition to Ms-phosphine iridium complexes [69]. 

All calculations were performed using Gaussian98 [77], which required making some additions and corrections to the UFF force field because Gaussian98 does not have atom types for square planar rhodium or coordinated double bonds. Details about these changes can be found in the supporting information of our original paper [72],... 

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