Protonation of the coordinated

Table I reports the observed NMR linewidths for the H/3 protons of the coordinating cysteines in a series of iron-sulfur proteins with increasing nuclearity of the cluster, and in different oxidation states. We have attempted to rationalize the linewidths on the basis of the equations describing the Solomon and Curie contributions to the nuclear transverse relaxation rate [Eqs. (1) and (2)]. When dealing with polymetallic systems, the S value of the ground state has been used in the equations. When the ground state had S = 0, reference was made to the S of the first excited state and the results were scaled for the partial population of the state. In addition, in polymetallic systems it is also important to account for the fact that the orbitals of each iron atom contribute differently to the populated levels. For each level, the enhancement of nuclear relaxation induced by each iron is proportional to the square of the contribution of its orbitals (54). In practice, one has to calculate the following coefficient for each iron atom ...

H2 activation by protonation of the coordinated allene in 8 with formation of the palladium hydride intermediate 9 and propene. 

In Eqs. (4)-(7) S is the electron spin quantum number, jh the proton nuclear magnetogyric ratio, g and p the electronic g factor and Bohr magneton, respectively. r//is the distance between the metal ion and the protons of the coordinated water molecules, (Oh and cos the proton and electron Larmor frequencies, respectively, and Xr is the reorientational correlation time. The longitudinal and transverse electron spin relaxation times, Tig and T2g, are frequency dependent according to Eqs. (6) and (7), and characterized by the correlation time of the modulation of the zero-field splitting (x ) and the mean-square zero-field-splitting energy (A. The limits and the approximations inherent to the equations above are discussed in detail in the previous two chapters. 

Only three PBP complexes of Fe(III) with acyclic ligands from the class of bis(acylhydrazone) of dap have been structurally characterized at present (41,49,53) and this prompted studies in that direction. Both direct and template synthesis afforded the complex [Fe(Hdapsox) Cl2] - 1/2H20 (9) with a monoanionic H2dapsox ligand (7). Protonation of the coordinated ligand was unsuccessful even upon addition of HC1 to the reaction mixture (Scheme 5). In spite of asymmetrical mono-deprotonation, the ligand was symmetrically pentadentately... 

The monomer species, M, has been described by Kraus (31) as an ion pair. Although he did not elaborate on its possible structure, one may assume that he pictured this species as two ammoniated ions held together by electrostatic forces. Douthit and Dye (12) pointed out that such a picture is consistent with the absorption spectra of sodium-ammonia solutions. Becker, Lindquist, and Alder (2) proposed an expanded metal model in which an electron was assumed to circulate in an expanded orbital on the protons of the coordinated ammonia molecules of an M + ion. The latter model is difficult to reconcile with optical, volumetric, and NMR data (16). 

In Eqs. (7-11), fi is the nuclear gyromagnetic ratio, g is the electron g factor, fiB is the Bohr magneton, rGdH is the electron spin - proton distance, co, and cos are the nuclear and electron Larmor frequencies, respectively (co=yB, where B is the magnetic field), and A/fl is the hyperfine or scalar coupling constant between the electron of the paramagnetic center and the proton of the coordinated water. The correlation times that are characteristic of the relaxation processes are depicted as ... 

Kinetic studies on the hydrolysis of [Ru(NH3)5NCO] to give [Ru(NH3)g] " and CO2 suggest that the reaction proceeds via N protonation of the coordinated NCO , followed by addition of HjO to give a carbamic acid complex, which subsequently loses HgO and CO2 to give the product (290). 

The parent group, HNO, was detected in the complex [OsCl2(HNO) (CO)(PPhj)2] formed through protonation of the coordinated nitrosyl group in [OsCl(NO)(CO)(PPh3)2] using hydrogen chloride. Carbon monoxide displaced HNO, but in solution the complex disproportionated with loss of HCl (243). 

The formation of these species, both of which probably coordinate a water molecule, is evident from the appearance of an intense yellow color of the solution. The bis-peroxorhenium complex is the most stable species. The protons of the coordinated water molecule are highly acidic, with consequences for the epoxidation reactions. 

The reversible protonation of the coordinated [OH] ligand in CAII (Figure 28.22a) is modelled by the reaction of complex 28.23 with (C5F5)3B(OH2) and subsequent deprotonation with Et3N (equation 28.18). The choice of acid is important as the conjugate base generally displaces the H2O ligand as in reaction 28.19. 

We also employed other ruthenium dihydrogen complexes for the protonation of the coordinated dinitrogen in complex (2b). Typical results are shown in Table 3 [21]. Ammonia is formed in higher yields (up to 79% based on tungsten) when dinitrogen complex (2b) is similarly treated with tra s-[RuCl(r -H2)(dppe)2]X... 

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