Trimers, hydroxide-bridged

From a structural point of view Bruno s hypothesis is sound. In chemical terms the hydrolysis proceeds with the formation of hydroxide bridges up to the formation of the trimer. Then at high concentrations trimeric units are fused together to form the higher aggregates, that can be seen as precursors to the structure adopted by Be(OH)2 in the solid state. (See Section IV, C, precipitation equilibria). Speciation diagrams for Bruno s model are shown in Fig. 3. Two points are noteworthy in these diagrams. In the first place the concentrations of the... 

The reaction rate of the reduction of these carbonyl compounds showed a sharp maximum at pH 3.2, which coincides with the pKA value of HCOOH in the studied concentration, and there was no reaction above pH 5. The lack of reactivity at higher pH can be attributed to the formation of the catalytically inactive hydroxide-bridged trimer, [(IrCp )2(p-OH)3]+ which, however, is in equilibrium with the starting catalyst precursor at the optimum pH of the reaction. The active form of the catalyst is most probably the dimeric [(IrCp )2(p-H)(p-HCOO)(p-OH)]+ which happens to form to the... 

We are faced, therefore, with the puzzling problem of why it is not possible to isolate any aquo-species and why all hydroxo-species are polymerized through hydroxide bridges. It is accepted that aquo-species exist in solution Raman spectroscopy (Figure 8) shows that the coordinated nitrate groups are replaced immediately on solution in water (32), and n.m.r. studies have shown that these aquo-species are more slowly converted into the polymerised dimer and trimer hydroxo-bridged species(30). 

The extraordinary stability of the UO + ion is also clear in its aqueous chemistry. If acid solutions are neutralised then, near pH 3.5, first dimers and then trimers are formed [10]. These have bridging hydroxide groups, but Raman spectroscopy shows that there is little change within the dioxo group as the condensation proceeds [11]. At pH 5 uranyl hydroxide is precipitated, the solid state structure confirming that the oxycation is intact [12]. 

As a structural model of the zinc(II)-OH species of carbonic anhydrase. the first mononuclear OH -bound zinc(TI) complex 6 was prepared by mixing zinc(II) ion. the ligand, and KOH. and was characterized by x-ray crystal analysis.The zinc(II)-OH complex lb is a cyclic trimer linked by three hydrogen bonds between each zinc(II)-bound hydroxide group, as shown by x-ray crystal studyP However, in aqueous solution, this trimer dissociates into monomeric zinc(II)-OH species lb. The zinc(II) complex 6 in CHCI3 reacted immediately with CO2, possibly to form a HCOa" complex 7 that ultimately and irreversibly gave a bridging carbonate complex 8. However, quantitative determination of the value for the zinc (II)-bound H2O or of the nucleophilicity of 6 was not reported. The HCOs complex 7 was characterized by IR spectroscopy [1675 and 1302 cm for zinc(n)-bound bicarbonate]. The formation of the bicarbonate com-... 

Einally, the caUx cavity is surprisingly reluctant to host anions in spite of the presence of the dicationic metal center. Adding halides or hydroxides in excess either leads to decoordination or it induces a rearrangement of the calixarene macrocycle to give dimers and trimers with the coordinated bridging anion outside the cavity. A likely explanation stands in the role of the oxygen-rich small rim of the host as illustrated in Eiguie 16.35,69,70... 

---