Monodentate bicarbonate

The hydroxy complex ria s-[Ir(OH)(CO)(PPh3)2] reacts with COj in ethanol to produce the monodentate bicarbonate complex [Ir(0C02H)((i 0)(PPh3)2]. The rate of reaction is st order in complex and independent of CO2 and H2O concentrations. The bicarbonate complex reacts with O2, affording [Ir(0C02H)(C0)(02)(PPh3)2]. ... 

Bands derived from monodentate and bidentate bicarbonate species were observed on Ga Oj after introduction of CO. The intensity of the absorbance of the band assigned to monodentate bicarbonate increased with the amount of introduced CO and became saturated after introduction of CO exceeded 2.5 kPa. In contrast, the absorbance of the band derived from bidentate bicarbonate species decreased with the amount of CO introduced and vanished after introduction of CO exceeded 2.5 kPa. The amount of chemisorbed CO gas was consistent with the amount of evolved CO gas, and these reached saturation at a CO equilibrium pressure of around 2.5kPa, as shown in Figure 12.5 this indicates that the chemisorbed CO species involved in the photocatalytic conversion of CO over is the monodentate... 

The photocatalytic conversion of CO in the presence of is proposed to proceed by an Eley-Rideal mechanism or Langmnir-Hinshelwood mechanism when carried out over typical basic oxides such as ZrO and MgO, or Ga203, respectively. In the case of ZrO and MgO, the intermediate derived from CO is a bidentate formate, whereas a monodentate bicarbonate species is formed in the case of Ga Oj. The bidentate formate intermediate formed on ZiO and MgO does not decompose directly to yield CO, but acts as a reductant and converts a second CO molecule to gaseous CO under photoirradiation. In contrast, the monodentate bicarbonate intermediate formed on Ga Oj decomposes directly. H, which acts as a reductant, cannot be adsorbed on ZrO and MgO but is dissociatively adsorbed on Ga Oj. Ga-H and Ga-OH species are formed via heteroly tic adsorption of Under photoirradia-... 

There has been particular recent interest in zinc nitrate complexes as coordination models for bicarbonate binding in carbonic anhydrase. The mono- or bidentate coordination modes have been studied with tris-pyrazolyl borate complexes and can be rationalized in the context of the enzyme activity.433 Caution in this comparison is introduced by ab initio calculations on these model systems demonstrating both monodentate and bidentate coordination energy minima for nitrate binding to zinc 434... 

A from cobalt. Hakansson and Wehnert (113b) described this residue of Thr-200-His variant of HCA II as pseudo bidentate coordination of bicarbonate to the metal. However, Tu et al. (167) pointed out from 180-exchange experiments that the bicarbonate bound to the metal is monodentate allowing proton transfer to the metal bound hydroxide (see Fig. 13). 

Cam suggest the availability of a continuum binding mode to the substrate, and perhaps isomerization of HCO3 could occur before its release. Kinetic studies also indicated that bicarbonate acts as a proton donor to the zinc-bound hydroxide during catalysis (219). Co-Cam co-crystallized with bicarbonate indicated that HCO3 binds monodentately to cobalt(II). 

CO2 and SO2 are frequently considered as acidic species. The adsorption of the former on MgO has been used to classify MgO as basic surface. The adsorption on 0 leads to C03 and SO3"" species that are nicely evoked to prepare the adsorbed species toward reactivity. For CO2, this leads to monodentate or bidentate carbonates and to hydrogen carbonates (bicarbonates). Monodentate carbonates have been postulated together with vertical C02[l]. On MgO[25] and Ti02 surfaces[26] the... 

A factor that can influence C02 hydration/dehydration reactivity is the overall coordination number of the zinc center and the coordination mode of a bicarbonate ligand (Fig. 7). It is reasonable to suggest that a unidentate coordinated HCO will be easier to displace, which could influence the rate of the overall hydration reaction. Data discussed below in terms of single turnover experiments supports the notion that bidentate bicarbonate coordination inhibits catalytic C02 hydration. Similarly, bidentate coordination of HCO could be expected to slow the dehydration reaction. Notably, X-ray crystallographic studies of bicarbonate-bound forms of a mutant CA-II, and a Co(II)-substituted form of the enzyme, have revealed both monodentate and bidentate coordination modes for the bicarbonate anion.28,32,45... 

Conversely, a marked influence of the Pd precursor used was noted. Indeed, on PdHT1.2(c) sample prepared using the chloride precursor, monodentate carbonates and bicarbonates were absent, while bidentate carbonates were formed in lower amounts than on catalysts of similar Pd loading prepared using Pd(acac) (compare curves 2 and 3 in Figure 1). 

In the calculated stucture of B [Zn(phen)2(0C02H)] (monodentate coordination of bicarbonate), a strong Zn-0(2) bond of 1.972 A and a weak binding of 0(1) to Zn (Zn-O(l) bond length 2.698 A) were reported, consistent with the corresponding Cu complex. It appears that the binding mode of bicarbonate in the Zn complex (B), assumes relatively more bidentate character than the same for the corresponding Cu complex which clearly shows monodentate coordination of bicarbonate 34k). 

The bicarbonate complex is a transient intermediate and rapidly aquates to HCO3 and the aqua complex due to the high substitutional lability of the monodentate ligand bound to Zn 73). The aqua complex [L-ZnOH2] , however, then undergoes a fast aqua-hydroxo equilibration. A similar study extended to the cyclen complex, Zn -[12]aneN4(M4), has provided the highest value of the hydration rate constant (/ cat = (3.5 0.1) x 10 s ), which is one-third of... 

At pH 7.0 and above only the carbonate surface complex was observed (75). Based on these results it appears that surface complexation modeling of carbonate should consider monodentate carbonate and bicarbonate bonding with... 

Theoretical work by Merz and co-workers" essentially follows the pathway in Scheme 8.7 and finds that the bicarbonate is bound in the Lipscomb model, with the Lindskog structure being an intermediate or transition state on the pathway to the more stable form. More recent theory" has emphasized the importance of H2O and other groups at the active site in facilitating the proton transfer reactions. It should be noted that the theoretical conclusions have changed and evolved with time" and must be viewed accordingly. Two X-ray structures with bicarbonate bound to CA I " and a CA II" mutant have been used to support the Lindskog structure. In simple models, both bidentate and monodentate HCOj" has been observed in Cu(II) complexes," while examples of bidentate Co(III)" and monodentate Ni(II)" coordination are known. These and other models have been reviewed recently." ... 

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