charge-transfer band, not observed experimentally. Nevertheless the problem is further complicated in that the position of the alg ( d t ) level depends critically upon whether the molecule is truly square planar with no near neighbors along the z axis or whether it is tetragonal with weak interactions along the z axis. Since the 1 :2 nickel complex, according to X-ray data,37v38 is N,N-coordinated forming five-membered rings, we could suggest a similar coordination for the ML species via the N atom of the amino group and the nitrogen atom of the deprotonated -NHO-group, as previously reported for nickel complexes of glycyl hydroxamic acid (aha)15*39 and seryl hydroxamic acid (adhp).30 The NiL2 complex being square planar, the small difference in log K for NiL and NiLz species [A log K (6.824 -6.476) = 0.3481 could be explained if we considered that it corresponds to an octahedral (NiL) to square-planar (NiLz) transformation. In addition, the logarithm of the second stepwise formation constant (log Kz) for the Ni2+-adhb system is appreciably greater than that of the first, log flIol. This is contrary to earlier findingsm3 using a method of calculation based on the erroneous assumption that the formation of NiL is effectively complete before NiL2 begins to be formed. The explanation of this behavior may lie in the ability of NiZ+ to form square-planar complexes; when a second ligand is bonding facially in a bidentate manner, the two five-membered (40) Braibanti, A.; Dallavalle, F.; Leporati, E.; Mori, G. J. Chem. SOC., Dalton Trans. 1973, 2539. (41) Braibanti, A.; Dallavalle, F.; Leporati, E.; Mori, G. J. Chem. Soc., Dalton Trans. 1973, 323. (42) Braibanti, A.; Mori, G.; Dallavalle, F.; Leporati, E. Inorg. Chim. Acta 1972, 6, 106. (43) Braibanti, A.; Dallavalle, F.; Leporati, E. Inorg. Chim. Acta 1969, 3, 459.Inorg. Chem. 1989, 28, 3759-3765 3759 chelate rings in the complex [NiL2] are much more stable than one five-membered ring in the [NiL] species. In conclusion the X-ray crystal structures of two complexes of n i~k e l ( I I )~~-~* and one of copper(II)eO containing two glycyl hydroxamate anions (L-= HzN-CH2-CONHO-) provide evidence of coordination by the nitrogen amino and the nitrogen of the hydroxamate moiety for adhb and ahpp, respectively. Additional evidence for this type of coordination, which persists in solution, was given by potentiometric and spectrophotometric studies on n i~k e l ( I 1 )~~~~~ and ~o p p e r ( I I ) .~~ We conclude therefore that M(I1)adhb and M-(11)-ahpp systems can probably satisfy different criteria for the biological activities and the analytical role proposed, giving a strong indication for the complexes investigated as being suitable sources of metal ions as trace elements that are essential in animal nutrition. Our results show clearly that, at physiological pH values, the assumption of an uncoordinated a-amino group in the M-(11)-adhb and M(I1)-ahpp systems is probably incorrect since the major species in this pH range is MLz, which involves coordination of the a-amino grou...
