General Considerations Concerning Metal Ions

By far the most common oxidation state among the lanthanides is + 3, although some + 2 ions have also been found. - For all the + 3 cations of the lanthanides, the electron configuration is identical, namely, 4f 5d 6s . First-row transition-metal ions, on the other hand, have variable oxidation states, dependent on their propensity to attain stable, orbital configurations, ranging from + 1 to +7 oxidation states. 

These comparisons should be taken into account when substituting lanthanide ions for first-row transition-metal ions in order to induce chemical shifts or change the relaxation behavior of a n.m.r. spectrum to elicit ion sequestering and structural information (see later). 

Paramagnetic Species Shift Reagents and Relaxation Probes 

Such n.m.r. shift reagents as Ni , Co, Fe + low-spin Fe, and lanthan-ide(III) ions (other than Gd, a relaxation ret ent) are characterized by having very fast electron-spin relaxation times 10 s), which pre- 

Pseudo-contact shifts can arise only in systems where the magnetic susceptibility of the paramagnetic center is anisotropic. This interaction appears to be dominant in certain transition-metal ions (Co ), as well as such rare-earth elements US Dy , Er +, and Yb. The chemi( shift from the dipo- 

---

Correspondingly, many papers have now appeared which concern metal ion (and especially Ca +) interactions with opiates, and each year a considerable number of new reports are published. This literature has been the subject of several reviews (JL- ). In the present article this body of work is considered in terms of the general hypothesis that neuronal Ca + levels control the nociceptive state and that opioids, both endogenous and exogenous, exert their main effects via changes in Ca + disposition. 

Many studies have been carried out concerning the stability constants of humic and fulvic acid complexes.188 190,191 Stability constants vary considerably with pH and ionic strength213 and this, together with the variable nature of the ligands involved, accounts for the range of values reported for individual metal ions in the literature. However, the stabilities of divalent metal complexes generally follow the well-known Irving-Williams order Mg < Ca < Mn < Co < Zn = Ni < Cu < Hg. 

---