6 and 7 is small relative to the first term so that y-= s> = Since Ka2 = 1.5 X << [H+], inspection of eq 8 shows that y+ will have a dependence on [Ni2+] and [ H + ] consistent with eq 4 if Ka" << [H'] and Kc is large enough or k13 is small enough so that the last term in eq 8 is not dominant. On the other hand, eq 9 predicts that y-should be independent of [Ni2+] because the last term in eq 7 disappears in the first-order approximation used to obtain eq 9. The magnitude of y-depends directly on k , , and k23, but the former may be established by y+ so that at least an upper limit on the value of kz3 is determined from y-. In addition, y-will not show much pH dependence if Ka' > [H+] and K / << [H+]. It is noteworthy that if Kc is decreased in eq 8, then k l , will decrease to maintain the magnitude of the last term and k 1 2 will increase to keep the first term constant. But the latter increase may be limited by the magnitude of y-. It should be apparent that it will be possible to evaluate k 1 2 + k 1 3 from eq 8, but the separation of the terms will depend on Kc in eq 8 and K , in eq 9.Qualitatively k , (y+) varies with [Ni2+] and pH as predicted by eq 8 and k , (7-) is relatively independent of these variables as required by eq 9. The values of k l (y+) and k2 (y-) have been fitted simultaneously by a nonlinear least-squares method to eq 5 while various constraints were imposed on the equilibrium constants, as indicated by the pH titration results. The experimental and calculated results are compared in Table 11. As expected from the above discussion, the value of k , , + kl3 is well-defined at (3.5 f 1.1) X IO5 M-I 8 , A "best fit" is obtained with K , + (Kc/K,") = 6 X lod and K, = 2.5 X lo", which gives k 1 2 = (1.9 f 0.7) ' lo5 M-' s-l and kl3 = (1.4 f 0.7) X lo5 M-I s-l, while k23 is undefined at 0.28 f 4.8 s-l. The latter value indicates that the intramolecular rearrangement path is not significant in the linkage isomerism process in this system. The values of k , , and kl3 are similar, as expected for a dissociative ion pair substitution on Ni(OH2)2+, and their magnitude is consistent with that expected for reaction of a dianion.* A series of Fe(CN)5L3-complexes, where L = adenosine, 1-methyladenosine, tubercidin, and 2-and 3-aminopyridines, were prepared and characterized in aqueous solution. A metal to ligand charge-transfer transition was observed for complexes of adenosine (337 nm), tubercidin (340 nm), 2-aminopyridine (345 nm), and 3-aminopyridine (362 nm). Their corresponding Fe(II1) complexes also display a ligand to metal charge-transfer absorption at 570 (adenosine), 635 (tubercidin), 655 (2-aminopyridine) and 695 nm (3-aminopyridine). The I-methyladenosine complex of pentacyanoferrate(I1) exhibits a band maximum at 375 nm, which is similar to that of imidazole complex. The rate constants of formation and dissociation were measured, and the kf and kd values (25 OC, p = 0.10 M (LiCI04), pH = 8) are 269 M-' s-I and 9.70 X s-I (adenosine), 257 M-' s-I and 0.612 s-I

Carbonyls' 2p orbital populations, [2p], in W(CO) 5 L {L = PPh 3 , PPh 2 Me, PPhMe 2 } have been determined by NMR spin-lattice relaxation techniques. Experimental values of axial [2p], compared with those reported for PMe 3 and P(OMe) 3 , reveal that PMe 3 is a slightly better p-acid than PPh 3 . Through space interactions between carbonyl and phenyl groups are insignificant since values of [2p] do not vary significantly in the series of phosphines, going from PMe 3 to PPh 3 . Natural bond orbital (NBO) studies indicate that p-accepting capabilities for these phosphines are primarily governed by the nature of P-C anti-bonding, s * P-C . Compared with PPh 3 , the better p-accepting s * P-C , as well as the better s-donating lone-pair LP(P), in PMe 3 can both be explained by the higher extents of rehybridization of the coordinated phosphorus atom. Based on this rehybridization argument, the NBO predicted order of increasing p-acidic strengths PPh 3 < PPh 2 Me < PPhMe 2 < PMe 3 , which cannot be clearly distinguished by NMR experiments, is ascribed to the same NBO trend of s-donating capabilities in a synergistic manner. Effects of coordination on P-Y (Y = C, O, F) bonding strengths in phosphines (or phosphites) are depending on two conflicting effects: rehybridization of LP(P) and the hyperconjugative-like d p ® s* P-Y back-donation.