Table 4 Rate comparisons for reductions by Ti(II) and Ti(III)a
b
b
Ox
Medium
kTi(II)
kTi(III)
[NDS•]2−
[H+] = 1.0 M; l = 1.0 M (HCl/CF3SO3H)
5.3 × 103
0.32
1.9 × 103
0.35
[H+] = 0.50 M; [I−] = 0.10 M; l = 1.0 M (HCl/LiCl/CF3SO3H)
[H+] = 0.50 M; l = 0.50 M (HClO4)
−
I3
Bzqnc
2.2 × 102
6 × 102
0.16
1.4 × 102
CHBzqnd
CoA5(H2O)3+e
CoA5F2+
CoA5Cl2+
[H+] = 0.50 M; l = 0.50 M (HClO4)
6 × 103
[H+] = 0.20 M; l = 0.50 M (HClO4/NaClO4/CF3SO3H)
[H+] = 0.50 M; l = 0.50 M HClO4
1.2 × 10−3f
1.4 × 102f
1.1 × 10−2f
250g
6.3 × 102
0.26
[H+] = 0.20 M; l = 0.50 M (HClO4/NaClO4/CF3SO3H
[H+] = 0.50 M; l = 0.60 M (HCl/NaCl/CF3SO3H)
3−
Co(C2O4)3
78g
a Reactions carried out at 22 ◦C unless otherwise indicated. b Rate constants in M−1 s−1. c 1,4-Benzoquinone. d 2,5-dichloro-3,5-dihydroxybenzoquinone
(II). e CoA5 = (NH3)5CoIII. f 25 ◦C; l = 0.50 M (LiCl) (G. A. K. Thompson and A. G. Sykes, Inorg. Chem. 1976, 15, 638). g Ref. 1.
(≥ 0.47 V) corresponds, in the Marcus treatmentfor outer-sphere
reductions,29 to a rate ratio kTi(II)/kTi(III) > 104.
4 J. W. Oliver and J. W. Ross, Jr., J. Am. Chem. Soc., 1963, 85, 2565.
5 H. Zimmer, D. C. Lankin and S. W. Horgan, Chem. Rev., 1971, 71,
229.
The much more modest selectivities observed in our systems
may imply an unusually low Ti(III,II) self-exchange rate in the
media at hand. Both TiII (3d2) and TiIII (3d1) are, in principle,
subject to minor Jahn–Teller distortions, but the extent of such
distortion and the occupancy of the non-degenerate orbitals
[(one short, two long) vs. (two short, one long)] may not
match. The two states might then have substantially different
geometries, and this mismatch would be expected to result in a
higher Franck–Condon barrier to exchange. An unusually low
self-exchange rate has been calculated for the analogous d1, d2
pair, [VIVO(H2O)4]2+, [VIII(OH)2(H2O)3]+.30
6 S. Yamada and R. Tsuchida, Bull Chem. Soc. Jpn., 1959, 32, 721.
7 H. Gehlen and J. Cermak, Z. Anorg. Allg. Chem., 1954, 275, 113.
8 E. S. Gould and H. Taube, J. Am. Chem. Soc., 1964, 86, 1318.
9 F. Basolo and R. K. Murmann, Inorg. Synth., 1953, 4, 171.
10 A. Haim and H. Taube, J. Am. Chem. Soc., 1963, 85, 495.
11 J. C. Bailar, Jr. and E. M. Jones, Inorg. Synth., 1939, 1, 7.
12 O. A. Babich and E. S. Gould, Inorg. Chem., 2000, 39, 4119.
13 B. A. Barshop, R. F. Wrenn and C. Frieden, Anal. Biochem., 1983,
130, 134.
14 Note that the quality of fit depends markedly on the ratio k−1/k2, but
not on the individual values of these parameters provided that k−1
is held between 0.005 and 1.0 M. Thus, this treatment allows us to
estimate the ratio k−1/k2 but not reliable values of k−1 and k−2
.
Our most remarkable result is doubtless the mechanis-
tic change in the Ti(II)–Co(III) reaction when the oxidant
[Co(NH3)5Br]2+ is substituted for its chloro analog. The nearly
linear profiles generated by reduction of the bromo oxidant when
Ti(II) is in excess imply that rates are determined by a process
involving the reductant but not the oxidant. This conclusion
15 A more complicated picture associated with reduction of the iodo
complex, which exhibits contributions both first- and second order
in Ti(II), is considered in a subsequent study.
16 D. A. Palmer, R. W. Ramette and R. E. Mesmer, J. Solution Chem.,
1984, 13, 673.
17 C. E. Johnson and S. Winstein, J. Am. Chem. Soc., 1951, 73, 2601.
18 A. Adegite and J. F. Lyun, Inorg. Chem., 1979, 12, 3602.
19 See, for example, A. G. Lappin, Redox Mechanisms in Inorganic
Chemistry, Ellis Harwood, New York, 1994, ch. 5.
20 Z. Yang and E. S. Gould, Dalton Trans., 2003, 2219.
21 R. M. Smith and A. E. Martell, Critical Stability Constants, vol. 3,
Plenum, New York, 1977, p. 282.
is closely related to that drawn for the Ge(II)–I3 reaction,12
−
and should also be considered exceptional since it defies the
accepted dictum23 that slow ligand-to-metal bond breakage in
coordination compounds occurs only if activation entails a
significant loss in ligand field stabilization.
Finally, it may be asked why evidence for a two-step mecha-
nism, i.e., (5)–(6), was not observed for Ti(II) reductions of the
fluoro- and chloro analogs of [Co(NH3)5Br]2+. We suspect that
although such a path was operative, it was overshadowed by the
more rapid (more usual) bimolecular path employed by these
oxidants.
22 See, for example: (a) A. H. Martin and E. S. Gould, Inorg. Chem.,
1975, 14, 873; A.H. Martin and E.S. Gould, Inorg. Chem., 1976, 15,
1934; (b) J. P. Birk, Inorg. Chem., 1975, 14, 1724.
23 F. Basolo and R. G. Pearson, Mechanisms of Inorganic Reactions,
2nd edn., Wiley, New York, 1968, Chaps. 1 and 6, Tables 1–9 and
6–4.
24 (a) J. P. Candlin and J. Halpern, Inorg. Chem., 1965, 4, 766; (b) M.-Y.
Wu, S. J. Paton, Y.-T. Fanchiang, E. Gelerinter and E. S. Gould,
Inorg. Chem., 1978, 17, 326.
25 S. K. Chandra and E. S. Gould, Inorg. Chem., 1996, 35, 3159.
26 Ti(III) is included here as its OH-bound cationic form in view of
the evidence that Co(III)–Ti(III) reactions proceed predominanantly
through the deprotonated reductant.
Acknowledgements
We are grateful to the National Science Foundation for support
of this work and to Mrs. Arla Dee McPherson for technical
assistance.
27 A. Haim, Inorg. Chem., 1968, 7, 1475.
28 W. J. James and J. W. Johnson, in Standard Potentials in Aqueous
Solution, eds. A. J. Bard, R. Parsons and J. Jordan, Marcel Dekker,
New York, 1985, ch. 18.
29 R. A. Marcus, Annu. Rev. Phys. Chem., 1964, 15, 155.
30 M. C. Ghosh and E. S. Gould, J. Am. Chem. Soc., 1993, 115,
3167.
References
1 Z. Yang and E. S. Gould, Dalton Trans., 2004, 3601.
2 U. Kolle and P. Kolle, Angew. Chem., Int. Ed., 2003, 42, 4540.
3 G. H. Forbes and I. F. Hall, J. Am. Chem. Soc., 1924, 46, 583.
1 7 8 4
D a l t o n T r a n s . , 2 0 0 5 , 1 7 8 1 – 1 7 8 4