Coordination mode carbonate

In another recent study, Mao et al. (34k) have used the Cu11 complexes of 2,2 -bypridine and 1,10-phenanthroline to identify the different carbonate coordination modes which may be kinetically significant. Stopped-flow and T-jump techniques were used. The... 

Cp(CO)2Re(THF) forms the complex 105 upon reaction with thiophene (89JA8753, 910M2436). Similar species are known for 2- and 3-methyl-, 2,5-dimethyl, and tetramethylthiophene (91IC1417). Thiophene in 105 is S-coordi-nated, and the sulfur atom is pyramidal. Treatment of 105 with Fc2(CO)9 produces 106, where the thiophene ligand is bridge-coordinated via the sulfur atom to rhenium and four carbon atoms of the dienic system with iron (the coordination mode). The pyramidal nature of the sulfur atom is preserved. The -coordination of thiophene separates the dienic and sulfur counterparts of the ligand and decreases the TT-electron delocalization, which leads to the enhanced basicity of the sulfur atom. 

Phospholes and analogs offer a wide variety of coordination modes and reactivity patterns, from the ti E) (E = P, As, Sb, Bi) through ri -dienic to ri -donor function, including numerous and different mixed coordination modes. Electrophilic substitution at the carbon atoms and nucleophilic properties of the phosphorus atom are well documented. In the ri -coordinated species, group V heteroles nearly acquire planarity and features of the ir-delocalized moieties (heterocymantrenes and -ferrocenes). 

Thirdly, it is significant that the difference in coordination modes of the bridging carbonate ligands of [TpPrl2]Zn 2(/u.-i71,rj2-C03) and [TpBut,Me]Zn 2(jLt-7j1,T71-C03) is manifested in their reactivity. Thus, whereas [TpButMe]Zn 2(/i-T71,i71-C03) reacts instantaneously with H20... 

The X-ray structure of the dibromine complex with toluene (measured at 123 K) is more complicated, and shows multiple crystallographically independent donor/acceptor moieties [68]. Most important, however, is the fact that in all cases the acceptor shows an over-the-rim location that is similar to that in the benzene complex. In both systems, the acceptor is shifted by 1.4 A from the main symmetry axis, the shortest Br C distances of 3.1 A being significantly less than the sum of the van der Waals radii of 3.55 A [20]. Furthermore, the calculated hapticity in the benzene/Br2 complex (x] = 1.52) is midway between the over-atom (rj = 1.0) and over-bond (rj = 2.0) coordination. In the toluene complex, the latter varies from rj = 1.70 to 1.86 (in four non-equivalent coordination modes) and thus lies closer to the over-bond coordination model. Moreover, the over-bond bromine is remarkably shifted toward the ortho- and para-carbons that correspond to the positions of highest electron density (and lead to the transition states for electrophilic aromatic bromination [12]). Such an experimental location of bromine is in good agreement with the results of high level theoretical... 

NMR measurements also provide information on the coordination of the ligands in the uranyl polymers. Solid-state I c-NMR confirms the coordination modes of the carboxylate ligands to the uranyl ion that is, both monodentate and bidentate carboxylate coordination modes are evident. The uranyl dicarboxyl ate polymers which possess two moles of coordinated DMSO exhibit two carbon-13 carbonyl resonances, one at about 175 ppm downfield from tetramethylsilane (TMS) and one at about 185 ppm. The polymers which possess only one mole of coordinated DMSO exhibit only the carbonyl peak near 185 ppm. Based on other known coordination compounds, the 175 ppm peak can be assigned to monodentate carboxylate and the 185 ppm peak to bidentate carboxylate. Thus, 7-coordination predominates in the polymers with either one or two moles of solvent coordinated to the uranyl ion, which is consistent with the infrared results reported elsewhere (5). 

The X-ray structure of the unsubstituted tris(pyrazolyl)borato zinc nitrate has been solved showing a unidentate coordination mode for nitrate, in contrast with the t-butyl substituted ligand, which shows anisobidentate nitrate coordination due to the steric effects.232 A partial explanation of the reduced activity of cadmium-substituted carbonic anhydrase is offered by Parkin on the basis of the comparison of nitrate coordination to cadmium and zinc trispyrazo-lylborate moieties. A contributing factor may be the bidentate coordination supported by the cadmium that does not allow the facile access to a unidentate bicarbonate intermediate, which could be highly important to carbonic anhydrase activity.233... 

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... 

In order to formally insert CH2, the growing alkyl chain must attain a situation of metal-to-carbon bonding that favors CH2 insertion over coupling of two CHR groups (R=H, alkyl).29 A (C, H)-chelating coordination mode characterized by agostic M-H-C interaction30 would meet this requirement. 

As anticipated by its ML2X2 formulation (Table 4.52), the computed structure of singlet nickelocene approximates a square-planar di-allylic coordination mode. We can deconstruct each r 3-Cp to Ni interaction into an electron-pair bond (M—X) with the radical carbon and a dative interaction (M—L) with the 7icc bond, symbolized as shown below with a half-filled circle ( >) to represent the radical site and a filled circle ( ) to represent the dative 7t-bond site ... 

The third ligand was assumed to be coordinated to the metal center via the deprotonated 3-hydroxy and 4-carbonyl groups. This coordination mode allows delocalization of the electronic structure and intermolecu-lar electron transfer from the ligand to Cu(II). The Cu(I)-flavonoxy radical is in equilibrium with the precursor complex and formed at relatively low concentration levels. This species is attacked by dioxygen presumably at the C2 carbon atom of the flavonoxyl ligand. In principle, such an attack may also occur at the Cu(I) center, but because of the crowded coordination sphere of the metal ion it seems to be less favourable. The reaction is completed by the formation and fast rearrangement of a trioxametallocycle. 

The potential carbodicarbene C(C(NMe2)2 2 has been known for a long time but no complex has been reported [101, 102]. It adopts a linear allene geometry in the free state but according to theoretical analysis exhibits a strong nucleophilic central carbon atom [4, 97] and can be seen as an allene with a hidden divalent carbon(O) character emerging in the presence of electron deficient electrophiles. Based on these findings a new field of chemistry will be opened and the number of compounds with a coordination mode should increase in the future. 

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