Bidentate carbonate

These hydrated salts contain bidentate carbonate ligands and no water molecules are bound directly to the central metal atom. The only single-crystal x-ray diffraction studies available are those for salts of (4) (52—54) and the mineral tuliokite [128706 2-3], Na2BaTh(C03)2 -6H20], which contains the unusual Th(C02) 2 anion (5) (55). 

Linear Bent1 Hydrogen carbonates Bidentate carbonates Monodentate carbonates Polydentate carbonates... 

The FTIR data recorded upon N0/02 adsorption in the presence of C02 showed that bidentate carbonates were immediately formed along with nitrite species. By increasing the time of contact, carbonates were partially displaced, while nitrites evolved to nitrate species [52], Notably, the amounts of surface nitrites present at each contact time are lower in the presence of C02 than in its absence. This indicates that C02 competes for the surface oxygen sites able to give nitrites at the beginning of the adsorption process in fact, after several minutes of exposure to the N0/02/C02 mixture, the amount of nitrates stored was comparable to that obtained in the absence of C02. In conclusion, while in the presence of C02 the nitrite route is inhibited to some extent due to the competition between NO and C02 for the surface oxygen sites of the Ba phase, the nitrate route is only marginally affected by the C02 presence, if any. 

Shinn and Eick 212) found that the orthorhombic crystals of La2(C03)3-8H2O contains two distinctive decacoordinated La(III) ions (space group Peon, Z—A, a =8.934, 6 = 9.580, c = 17.00 A). The coordination polyhedron of La is made up of ten oxygen atoms from water molecules, bidentate carbonates and xmidentate carbonates. One fourth of the water molecules in this compound are not bonded to the metal ion but these are situated between layers. The coordination polyhedron is assumed to be distorted Civ The oxygens of the bidentate carbonate ion form longest La—O bond [La(l)—0(bident. CO3) =2.73 A, La(2)— O (bident. C03)=2.74A], whereas the water molecules are nearer to the lan-thanium ion (La—OH2 =2.63 A). 

Pospelova and Zaitsev393 have reported a series of basic carbonate complexes of zirconium in which the metal-to-carbonate ratio varies from 1 1 to 1 3. Typical compositions are M Zr20(0H)6 B(C03)n(H20), (n = 2, 3, 4 or 6) and M Zr20(C03)5(H20), (M =Na, K, NH4, C(NH2)3, etc.). Analogous carbonato-oxalato complexes such as M4Zr20(C03) (C204)6- (H20)x (n = 2 or 4) are also known.396 IR studies have shown that the basic carbonate complexes contain bidentate carbonate ligands,39S but only the 1 3 complexes... 

Ceric ammonium nitrate, the well-known volumetric standard oxidant, is formulated (NH4)2[Ce(N03)6] with 12-coordinated Ce4+ ions (each N03 ion being bidentate) in approximately Td symmetry. The Ce—O distances are in the range 2.488-2.530 A. The ligand bite is only 50.6-50.9° and the other O—Ce—O angles are 65.0-69.90.692 The carbonato complexes Na6[Ce(C03)5]-12H20 and [C(NH2)3]6[Ce(C03)5]-4H20 each show cerium ten-coordinated to five bidentate carbonate ions with Ce—O = 2.379-2.504 A in the first complex and 2.388-2.488 A in the second. The coordination polyhedra are irregular.693,694... 

Less is known about the combustion pathway. Using IR spectroscopy, formate, carboxylate, bidentate carbonate, monodentate carbonate, and acetate bands have been observed when butene is adsorbed on Surface degradation (oxidation) of butadiene has also been observed on a-Fe203 at room temperature (34). 

He found that below 573 K, the rate of surface reduction of C03O4 is much slower than the rate of CO. oxidation. Thus, participation of Ol in CO oxidation at 488 K is unlikely. If Ol were consumed in CO oxidation at steady state, one would expect regeneration of 0 to be fast. Experimental evidence suggests the opposite, l.e. that catalyst regeneration is slow. Formation of mono-dentate carbonate ([Ol] C02 in step 2) along with bidentate carbonate had been spectroscopically observed on C03O4 surfaces upon CO adsorption by Hertl (6) and Goodsel (7). Formation of these species rather than carbonyl type species indicate the roles played by O and 0qo. Step 2 could proceed via an intermediate step in which bidentate carbonate is formed as shown ... 

Vibrational mode C02 (gas) (cm-1) Carbonate ion (cm -1) Unidentate carbonate (cm-1) Bidentate carbonate (cm-1) Organic (bridging) carbonate (cm-1) Bicarbonate (cm-1)... 

Carbon dioxide adsorption by highly dehydroxylated titanium dioxides gives rise to bands that are best ascribed to a bidentate carbonate species. The corresponding band pairs were reported to appear at 1580 and 1320 cm-1 by Yates (132) and O Neill and Yates (268) and at 1584 and 1375 cm-1 by Primet et al. (176, 269) on anatase and at 1485 and 1325 cm-1 on rutile (132). These bidentate carbonate species are stable during pumping. 

Carbon monoxide adsorption on ZnO has been found to depend strongly on the pretreatment of the oxide. Kortiim and Knehr (94) detected a weakly bound CO with at least one rotational degree of freedom showing rotational-vibrational bands with maxima at 2188 and 2120 cm-1 and a strongly bound complex similar to bidentate carbonate. In addition, well-degassed ZnO specimens adsorbed CO with the IR frequency within the carbonyl... 

The octahedron also is adopted in the known carbonates249 (Table 22). MnC03, although thermodynamically unstable in 02 or air, decomposes only slowly, and the pink, almost white, compound occurs naturally as the mineral rhodochrosite. Only one crystalline form is known which has the calcite structure and both CaMn(C03)2 and BaMn(C03)2 have the dolomite structure. The carbonate mineral sidorenkite (Na3MnC03P04) has a bidentate carbonate group, which is somewhat unusual in inorganic structures. 

Few well-defined Con-carbonate complexes are known. The two structures given in Table 68 are probably representative of chelated and monodentate carbonate. K2[Co(C03)2(H20)4] crystallizes as rose-coloured plates from warm solutions of Co 1 salts in the presence of K2C03s7° and contains trans monodentate carbonate. Bidentate carbonate in [Co(C03)(H20)2(imH)2] appears to exert a measurable structural tram influence on the trans water molecules (CoO, 216.6 pm vs. 209.4 in Co(H20)5+)330 and a similar effect is found in Coin-carbonato complexes. Violet crystals of [Co(C03)(H20)2(NHpy2)] have been isolated on reacting Na3[Co(C03)3] with 2,2 -bipyridylamine and presumably contain chelated carbonate (IR n = 5.23 BM).871... 

AH values for the decarboxylation of the bidentate carbonate, imply that the first, rate-determining step in this reaction must involve cleavage of a Rh—O bond as the carbonate ring is opened, and rapid decarboxylation of the monodentate complex can then occur with only C—O bond cleavage. 

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... 

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