PROTON-EXCHANGE AND INVERSION AT COORDINATED SEC-AMINE CENTERS - PARTICULARLY RAPID INVERSION AT STERICALLY STRAINED PLANAR N IN SOME [CO(CYCLEN)(S-ALAO)](2-ALAO)](2+) COMPLEXES() AND [CO(MECYCLEN)(S)

It is shown that the syn(N),syn(O)- isomer (3) is the intermediate on the reaction path for interconversion between the syn(N),anti(O)- (1) and syn(O),anti(N)- (2) diastereomers of syn(Me)-[Co(Mecyclen)(S-AlaO) ](2+) and [Co(cyclen)(S-AlaO)](2+). H-1-exchange rate constants for th e various NH protons are reported (k(H)/M(-1)s(-1), 25 degrees C, D2O) and are correlated with the derived isomerization rate constants (k(N )/M(-1)s(-1), 25 degrees C, H2O). At equilibrium (25 degrees C, I simi lar to 0.1 M) the [1]:[2]:[3] ratios are 73:21:6 and 63:32:5, respecti vely, for the two complexes. Ratios of k(H)/k(N) show that the two ''p lanar'' sec-N centers of the coordinated cyclen ligand are particularl y susceptible to inversion, with estimated k(2) values for lone pair i nversion being between 5 x 10(6) and 2 x 10(8)s(-1) at 25 degrees C. T he effects of long-range electronegative substituents (trans carboxyla te-O, trans amine-N) and CNC bond angle strain on inversion at metal-c oordinated amines is discussed.