Equilibrium constants hydride complexes

Since the nature of the hydride chemical shifts, particularly in transition metal hydride complexes, is not simple [32], there is no reliable correlation between Sh and the enthalpy of dihydrogen bonding. Nevertheless, the chemical shifts of hydride resonances and their changes with temperature and the concentration of proton-donor components, for example, can be used to obtain the energy parameters for dihydrogen bonding in solution. As earlier, the enthalpy (A/f°) and entropy (AS°) values can be obtained on the basis of equilibrium constants determined at different temperatures. Let us demonstrate some examples of such determinations. 

Table III also shows the values of the equilibrium constants, KVAp for the conversion of iron nitrosyl complexes into the corresponding nitro derivatives. Keq decreases downwards, meaning that the conversions are obtained at a lower pH for the complexes at the top of the table. Thus, NP can be fully converted into the nitro complex only at pHs greater than 10. The NO+ N02 conversion, together with the release of N02 from the coordination sphere, are key features in some enzymatic reactions leading to oxidation of nitrogen hydrides to nitrite (14). The above conversion and release must occur under physiological conditions with the hydroxylaminoreductase enzyme (HAO), in which the substrate is seemingly oxidized through two electron paths involving HNO and NO+ as intermediates. Evidently, the mechanistic requirements are closely related to the structure of the heme sites in HAO (69). No direct evidence of bound nitrite intermediates has been reported, however, and this was also the case for the reductive nitrosylation processes associated with ferri-heme chemistry (Fig. 4) (25).

When styrene is added to HNi[P(0-o-tolyl)3]3CN, the solution goes from yellow to red and the hydride is quantitatively converted to alkyl complex. However, addition of excess styrene to HNiL3CN—B(p-tolyl)3 causes a color to change to orange and leaves most of the nickel as hydride complex, as shown by both IR and NMR spectroscopy. Thus, the Lewis acid decreases the equilibrium constant for reaction (26) relative to reaction (25). 

While the mechanism for the formation of the methoxy complex (14) is not established, it is significant that the dihydride Zr(C5Me5)2H2 is needed for the reduction of the CO coordinated in (13). A reasonable proposal for this reaction can be formulated if it is assumed that since complex (13) is formally d°, Zr—CO backbonding will not be of major importance, and that hydride complexes of the group 4 elements possess substantial hydridic character. The first assumption may lead to a more favorable equilibrium constant for carbonyl hydride formyl interconversion as in (5), while the second suggests H" attack in this sequence presumably on a coordinated formyl. If the latter results in Zr—H addition across C=0, then reductive elimination of a C—H bond leads to the observed product. This is shown in (21). 

A complete kinetic scheme has been established for the enzyme from both sources. The L. casei dihydrofolate reductase followed a reaction sequence identical to the E. coli enzyme (Scheme I) moreover, none of the rate constants varied by more than 40-fold Figure 20 is a reaction coordinate diagram comparing the steady-state turnover pathway for E. coli and L. casei dihydrofolate reductase, drawn at an arbitrary saturating concentration (1 mM) of NADPH at pH 7. The two main differences are (i) L. casei dihydrofolate reductase binds NADPH more tightly in both binary (E-NH, -2 kcal/mol) and tertiary (E NH-H2F, - 1.4 kcal/mol E-NH-H4F, - 1.8 kcal/mol) complexes, and (ii) the internal equilibrium constant (E-NH H2F E-N-H4F) for hydride transfer is less favorable for the L. casei enzyme (1 kcal/mol). These changes, as noted later, are smaller than those observed for single amino acid substitutions at the active site of either enzyme. Thus, the overall kinetic sequence as well as the... 

The equilibrium constants determined in Eqs. (8) and (9) are actually Boltzmann factors (statistics not included), but they indicate that in this case the heavier isotope prefers to occupy the hydride site. In contrast, both Field and Crabtree had previously observed the opposite effect, e.g., = 1.3 for ReD2(HD) - ReHD(D2) in [ReH2(H2)(CO)(PMe2Ph)3]+.57,58 However, detailed analysis of chemical shift and coupling constant data as a function of temperature in the Ir complex has not been carried out in these systems. The isotope effect was simplistically interpreted to be a consequence of a greater vibrational zero-point energy difference between Re(i/2-HD) and Re(f/2-D2) relative to Re-H and Re-D.58 These systems are quite complex... 

A difference from previous studies of metal hydride complexes is that we have created a continuous ladder of overlapping acid/base equilibria. This involved finding a series of phosphoms-containing compounds with pK that differ by less than 2 units, the limit of accurate determination of equilibrium constants K. We have determined the equilibrium constants, K, for cationic acid- neutral base reactions (eq 13) or neutral acid-anionic base reactions (eq 14) in THF or THF-dg by use of quantitative 3 P gated H and H NMR. 

A different thermodynamic cycle (Scheme 3.8 and Equation 3.129) can give the free energy of H dissociation from a hydride complex. In some cases the results (Table 3.5) - have been confirmed by measuring the equilibrium constant for heterolytic cleavage of Hj by and a base B. 

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