Hydroxycarbonyl complex

In this context we postulated that the shift reaction might proceed catalytically according to a hypothetical cycle such as Scheme I. There are four key steps in Scheme I a) nucleophilic attack of hydroxide or water on coordinated CO to give a hydroxycarbonyl complex, b) decarboxylation to give the metal hydride, c) reductive elimination of H2 from the hydride and d) coordination of new CO. In addition, there are several potentially crucial protonation/deprotonation equilibria involving metal hydrides or the hydroxycarbonyl. The mechanistic details have been worked out (but only incompletely) for a couple of the alkaline solution WGSR homogeneous catalysts. In these cases,... 

Addition of excess base in these reactions generates the dianionic starting material in Eq. 15.16. The intermediate shown in Eq. 15.19, which has a coordinated formyl group (C(O)H), has been isolated and characterized. Its counterpart in the OH reaction is a hydroxycarbonyl complex (see Problem 15.45) Both are inherently unstable, decomposing to the hydrido complex by elimination of CO or CO,. When these reactions are attempted with Cr(CO)fc, the final product is not the expected [HCriCO) ] anion, but rather a bridging hydride complex "... 

Organometallic complexes frequently are susceptible to nucleophilic attack by an external reagent. In some instances the attack takes place on the metal center (see substitution reactions, page 686). while in others it occurs on a bound ligand. Already in this chapter we have seen many instances in which coordinated carbon monoxide undergoes nucleophilic attack. Examples include reactions with H to produce a formyl complex (Eq. 15.19). with R to form an acyl complex (Eq. 15.49). and with OH to give a hydroxycarbonyl complex (Eq. 15.21). 

It is amphoteric, being deprotonated by xs KOH while dissociating hydroxyl ion [i.e., reversing Eq. (g)] in solvents of high dielectric constant such as formamide. The neutral hydroxycarbonyl complex loses COj on warming, whereas its potassium salt is stable in solution even at lOO C ... 

These ruthenium M-CO and M-COj complexes are the first complexes which interconverse each other both in protic and aprotic solvent. The comparison of eqs. 6 and 7 indicates that the role of CO3 - and CO2 in acetonitril solution corresponds to that of OH- and H in aqueous solution, respectively. The proposed head-to-tail adduct corresponds to hydroxycarbonyl complex in which the position of the cleft bond determines the product when the C(0)-0H bond cleaves, the M-CO complex is generated, and when the C(0)0-H bond cleaves, the M-CO2 complex is generated (eq. 7). The interconversion of RU-CO2 and Ru-CO in protic... 

An alternative preparation method for metal TjHCl-COa complexes is the deprotonation of hydroxycarbonyl complexes 25). A general route for the preparation of hydroxycarbonyl complexes is a nucleophilic attack of OH" or H2O on cationic metal carbonyl species (52) ... 

The first stable hydroxycarbonyl complex [IrCl2L2(C0)(C(0)0H)] (L = PMe2Ph and AsMe2Ph) was obtained by hydrolysis of [IrCl2L2(CO) (C(O)OMe)] (53). A specific reaction is CO insertion into the metal-... 

C0)(PPh3)(C(0)0H)] with excess of KOH (24, 54). The wide range of p/iia values for hydroxycarbonyl complexes (Table II) may reflect the degree of electron transfer from the metal to CO2. 

OH ), is enhanced by protonation of the central metals in acidic conditions. The WGSR catalyzed by [Rh(bpy)2(CO)] (59) has been shown to proceed via the formation of the hydroxycarbonyl complex assisted by protonation of Rh (Scheme 3). 

The importance of hydroxycarbonyl intermediates is well illustrated in a recent study of the stepwise oxidation of CO to CO2 by binuclear ruthenium complexes". Deprotonation of a diruthenium(I) aquo species yields a hydroxy intermediate which rearranges to an isolable hydroxycarbonyl complex, equation (h). Deprotonation of the hydroxycarbonyl with NEts in dichloromethane results in a formally diruthenium(O) ii-CO complex, equation (i). 

The hydroxycarbonyl complex generated by the attack of OH on the coordinated CO (Scheme 1.34a) is susceptible to decarboxylation affording a metal hydride, which may react with a proton to produce H2. The process is considered to be involved in catalytic conversion of CO and H2O into CO2 and H2 (the water gas shift reaction) as shown in Eq. 1.17. 

Hydroxide ion attacks metal-carbonyl complexes to form hydroxycarbonyl complex-es. These complexes are generally unstable and extrude COj to produce metal-hydride complexes (Equation 11.4). When the anionic hydride is sufficiently acidic to be deproto-nated by the hydroxide, an overall two-electron reduction of the metal results. Thus, CO... 

In basic medium, in the first step, nucleophilic attack of OH on the coordinated CO molecule occurs and, as a result, a hydroxycarbonyl complex (metallocarboxylic acid) is formed. Such compounds are formed during the conversion of CO (water-gas shift reaction, wgsr). ... 

An interesting feature of the chemistry of monomeric organoplatinum(II) hydroxides and alkoxides is that CO inserts into the Pt—O bond more readily than into the Pt—C bond. Thus in 1993, Bennett and coworkers in an effort to examine the structures and properties of Pt(II)-hydroxycarbonyls with trans-arranged phosphines explored the reaction of the monomeric pincer Pt(II)-hydroxo species (14) with CO [9]. Hence, treating the (PCP)Pt(OH) pincer complex (14) with CO in benzene under ambient conditions caused the formation of the hydroxycarbonyl complex (15) as a... 

Electrocatalysis by transition metal complexes is elegantly illustrated by the work of DuBois and colleagues.2 In the absence of CO2, the palladium (II) complex undergoes two-electron reduction, but when CO2 is present, the one-electron reduction product binds CO2 (DMF as solvent) (Figure 5.9). With added acid, carbon monoxide is produced catalytically from carbon dioxide. This chemistry likely involves the protonated carbon dioxide adduct (hydroxycarbonyl complex) shown in Figures 5.9 and 5.10. Catalytic turnover numbers greater than 100 have been reported for this and related compounds. Some hydrogen is produced in parallel, evidently via a hydride complex. 

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