Co(dppe)2] would be expected to have a Z)M structure lying between square planar and tetrahedral. In contrast, PP3 would be expected to impose a pyramidal structure on [Co(PP3)] as is observed for [Ni(PP3)](C104).21 Since the pyramidal geometry is energetically less favored than a Z)M geometry, the corresponding reduction would be expected to be more difficult.For nickel(II), the PP3 ligand stabilizes a five-coordinate trigonal-bipyramidal structure, while dppe and PP2 form fourcoordinate square-planar complexes. For all three nickel complexes three oxidation states are available in the potential range of -0.7 to -1.3 V vs. ferrocene, and all exhibit a reversible or quasi-reversible one-electron wave for the Ni(II/I) couple. The Ni(I/0) couple is reversible for [Ni(dppe)2](BF4)2 and irreversible for [Ni(PP3)(CH3CN)](BF4)2 and [Ni(PP2)(CH3CN)](BF4)2.For [Ni(PP3)(CH3CN)](BF4)2 the irreversibility of the Ni(I/0) couple is due to a ring opening process in which a metal-phosphorus bond is broken and a Ni(0) dimer, [Ni(PP3)]2, is formed. The rupture of the nickel-phosphorus bond is attributed to ring strain in the transient pyramidal [Ni(PP3)J complex. In contrast, [Ni(dppe)2](BF4)2 is free to distort to a tetrahedral structure on reduction, and the Ni(I/0) couple is reversible. For [Ni(PP2)-(CH3CN)](BF4)2 the irreversibility of the Ni(I/0) couple probably arises from loss of acetonitrile on reduction to Ni(0).

The luminescence lifetimes of N-deuterated Cr(III) complexes of macrocyclic tetraamine ligands, trans-CrN(4)X(2)(n)()(+), are substantially longer than those of their undeuterated counterparts in room temperature solution. Thus, excited-state emission quenching of the longer lived species by the shorter lived species may be studied by analyzing the decay profile following pulsed excitation. Flash photolysis experiments were carried out for three deuterated/undeuterated pairs of trans-CrN(4)X(2)(n)()(+) complexes (where X = CN-, NH(3), and F-). For the trans-Cr(cyclam)(CN)(2)(+) system in H(2)O, it was determined that energy transfer was occurring between the deuterated and undeuterated species. Although the rate constant of energy transfer was too fast to measure explicitly, it could be bracketed as k(et) >>7 x 10(6) M(-1) s(-1). For this reaction it was possible to measure an equilibrium constant which was very near 1.0. For trans-Cr(cyclam)(NH(3))(2)(3+) in DMSO, it was also established that energy transfer was occurring and rate constants of 2.4 x 10(6) M(-1) s(-1) (mu = 0.1) and 9.7 x 10(6) M(-1) s(-1) (mu = 1.0) were determined by a Stern-Volmer analysis. For trans-Cr(tet a)F(2+) in H(2)O, no energy transfer was observed, which implies that the rate constant is <<3 x 10(5) M(-1) s(-1). Because these energy-transfer reactions represent self-exchange energy transfer and are thus thermoneutral, we are able to analyze the results using Marcus theory and draw some conclusions about the relative importance of nuclear reorganization and electronic factors in the overall rate.

and MY"*] » ML1 234567*] are met. At this point, Md will approach the value of k-L, where M is the limiting rate of dissociation of L"4" from Ru(CN)5L(3"'i)•. Similarily, as [Ym+] is decreased, the observed rate constant approaches the rate of dissociation, M> of Y"4" from Ru(CN)5Y(3~'"h The expression in eq 5 is consistent with the observed dependence of the rate on [Ym+] for the substitution reactions in this study. With k_L for the complexes with nitrogen heterocycles being greater (Table I) than the corresponding rate constant for the dimethyl sulfoxide complex, fcobsd increases to a limiting value (Figure 1) with increasing [N^SO],In the substitution reactions of the dimethyl sulfoxide complex, an increase in [Ym+] results in a decrease in kobsi to a limiting value.Rate constants and activation parameters have been reported for formation and dissociation reactions of the analogous Fe-(CN)5L(3~',,~complexes. [1][2][3][4][5]14 The enthalpies of activation associated with M are similar for the two systems while the entropy values are slightly lower (5-10 cal K"1 mol"1) for the ruthenium complexes. The decreased entropies of activation are suggestive

Starting from the trans‐dichloro perchlorate Complexes of the macrocyclic title ligand such as (Id), the trans‐Cr(III) and trans‐Rh(III) Complexes (lac) and (Ie) are prepared by anion exchange.

248ChemInform Abstract The complexes (I) and (II) are prepared with 85% and 62% yield by refluxing the corresponding macrocycles with trans-(Cr(Py)4F2)ClO4 in methoxyethanol. The photobehavior of the complexes is investigated and compared with that of analogous nonmacrocyclic counterparts.