Evidence that closely coupled porphyrin-related structures are involved in bacterial1 or green plant2 photosynthesis has led to much recent work on such associated systems in vitro, in which a number of covalently linked porphyrin "dimers" or higher complexes have been synthesized by various means.3 We now

Ministero della Pubblica Istruzione is gratefully acknowledged. AppendixGenerally, the diffusion and dissociation rate constants kd and are obtained from the Debye66 and Eigen67 equations kd(n = 0) 8 RT * with Sr = 0.8 A,7 one obtains for k and AG* values that are within the uncertainty given in the text.

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Stability constants for lanthanide and alkaline earth cryptate formation have been measured in (CH,)2S0 by visible spectroscopic techniques using murexide as a metal ion indicator. Three cryptands (222, 221, and 211) were examined with five lanthanide ions (Pr3+, Nd3+, Gd3+, Ho3+, and Yb3+) and three alkaline earth ions (CaZ+, Sr2+, and Ba2+). Lanthanide complexes with 21 1 are more stable than those of the larger ligands. No appreciable metal ion specificity is observed in the formation of lanthanide cryptates, but the alkaline earth ions display metal ion specificity similar to that observed in water. The stability constants in (CH3)2S0 are less than those measured in H 2 0 by at least 2 orders of magnitude for all of the ions studied. Also, addition of (CH3),NC1 leads to lower apparent stability constants in the case of lanthanide cryptates but has little effect on the formation of alkaline earth cryptates. Reasons for these results are presented.Cryptands are macrobicyclic ligands that form inclusive complexes with various metal ion^.^-^ Interest in these ligands A recent study (Anderegg, G. Helu. Chim. Acta 1981,64, 1790) shows Ce3+C222 (log K = 8.4) to be less stable than La3+C222 (log K = 9.4) in 9 5 5 methanol-water. Attempts to measure stability constants for other lanthanide ions and other cryptands were unsuccessful due to metal ion hydrolysis. The relatively large values of the measured constants are consistent with cryptate formation in methanol and methanol-water solvent^.^^*^ Since results are reported only for two ions at the beginning of the lanthanide series, it is not yet clear whether any significant metal ion specificity exists in this solvent. Cahen, Y. M.; Dye, J. L.; Popov, A. I. J. Phys. Chem. 1975, 79, 1289. Abraham, M. H.; Viguria, E. C.; Danil De Namor, A. F.; Hill, T. Inorg.With use of potentiometric pH and pAg measurements the stability constants and the stoichiometric composition of the Ag(1) complexes with some N-methyl-substituted 4-methyldiethylenetriamines in aqueous solution in a medium of ionic strength 1.3 and at a temperature of 25 OC have been determined. It is found that, besides mononuclear complexes, polynuclear and protonated complexes are formed. There is evidence for the formation of hydroxo complexes. The values of the stability constants are discussed in terms of possible structures. The mean amounts of complexed Ag' , H+ or OH-, and L are calculated, by using pH and pAg data and without making any assumption about the stoichiometric composition of the formed complexes.