We hope that this Review has made readers more aware of solvation of inorganic complexes, and of the importance of such knowledge in understanding their chemistryperhaps particularly their reactivity. The approach just set out for inorganic complexes should be of considerable value in the field or organometallic chemistry. In particular, informed use of solvation characteristics should help in optimising conditions for organometallic reactions and in homogeneous catalysis. Unfortunately, solvation data on reactants are too sparse (the subject index ofComprehensive Organometallic Chemistry contains justthree entries under “solubility”!) for serious examination of reactivity trends in terms of initial state and transition state contributions to be possible in almost all areas. Moreover, there are some fundamental problems over transfer parameters. Thus, a favourite electrochemical assumption is that the ferrocene/ferrocinium redox potential is independent of solvent. Yet, the dependence of rate constants on medium for outer-sphere electron transfer in the ferrocene/ferrocinium system can only be understood(66) in terms of specific solvation effects which are incompatible with the parallel solvation changes of these two substrates implicit in the redox potential assumption. The solvation of organometallic species should prove a most rewarding area for continued study, but it will be some time before the overall picture becomes as clear as in the more limited area of classical transition metal complexes considered in the present Review.

Solvation of transition metal complexes: thermochemical approaches

The solvent, pressure, and temperature-dependence of the lowest energy metal-to-ligand charge-transfer absorption bands are reported for a series of [Mo(CO)4(LL)], compounds where LL is a bidentate diimine ligand containing the chelating fragment −N=CRCR=N−, and one terdentate ligand analogue [Mo(CO)3(LLL)] The effects of ligand nature on these dependences are discussed, as are their interrelations and their connection with solvent properties such as polarity

Solvent, ligand, pressure, and temperature effects on charge-transfer spectra of tetracarbonylmolybdenum(0) diimine complexes

Piezochromic and thermochromic behaviour of ternary diimine–cyanide–Iron(II) and –Iron(III) complexes

Though 2,2′:6′,2′′-terpyridine is commonly used as a ligand for forming metallo-supramolecular assemblies in a variety of solvent environments, little is known about how the properties of the medium influence its behavior. We have studied the kinetic stabilities of bis terpyridine complexes with iron(II) and cobalt(II), and found variations in decay rate between commonly employed solvents of more than five orders of magnitude for iron, and thirty-fold for cobalt. While numerous factors likely contribute to these effects, a clear correlation between the decay rates of the iron complexes and the solubility of terpyridine suggests that favorable solvent–ligand interactions may lower the activation barrier for dissociation by stabilizing the transition state. These results illustrate the importance of solvent selection when designing metallo-supramolecular assemblies based on terpyridine, and also provide insights for further tuning the dynamic properties of these complexes.

Kinetic stabilities of bis-terpyridine complexes with iron(II) and cobalt(II) in organic solvent environments

Kinetics of the base hydrolysis of several iron(II)–diimine complexes, including two with terdentate ligands and one with an encapsulating ligand, and of the peroxodisulfate oxidation of three ternary iron(II)–diimine–cyanide complexes, are reported for reaction in dimethyl sulfoxide (DMSO)–water mixtures at 298.15 K. Solubilities of simple and complex salts have been determined in these mixtures, both to extend the range of simple and complex ion-transfer chemical potentials and to provide the basis for initial-state/transition-state analyses of reactivity trends for several of the base hydrolysis and peroxodisulfate oxidations. Cases are identified where the destabilisation of hydroxide ions, OH–(aq), by added DMSO increases the rate of base hydrolysis. We also identify cases where compensation effects result in only a small change in rate constant for oxidation by peroxodisulfate where DMSO is added.

Solvation and reactivity of iron(II)–diimine complexes in dimethyl sulfoxide–water mixtures

The dependence of the charge-transfer frequency for [Mo(CO)4(btz)], btz = 2,2′-bi(4H-5,6-dihydrothiazine), on solvent is described, and the solvatochromic behaviour of this compound compared with that of other [Mo(CO)4(LL)] species, with LL = 2,2′-bipyrimidine or 2,2′-bipyridine, and of iron(II) analogues [Fe(btz)2(CN)2] and [Fe(bipy)2(CN)2]. Kinetics of solvolysis (k, ΔH≠, ΔS≠) are reported for [Mo(CO)4(btz)] in methanol, acetonitrile, and dimethyl sulphoxide. These kinetic results are analysed into initial state and transition state contributions. A parallel analysis of the solvatochromic results for [Mo(CO)4(btz)] into ground state and excited state solvation contributions is compared with similar analyses for the solvatochromic organic compoundsp-nitroanisole and dimethylindoaniline.

Reactivity and solvatochromism of 2,2′-bi(4H-5,6-dihydrothiazine)-tetracarbonylmolybdenum(O)

Rate constants for racemisation and dissociation of the tris(1,10-phenanthroline)iron(II) cation, and solubilities of its picrate, are reported for dimethyl sulphoxide-water mixtures (0 to 40% by vol DMSO) at 298.2 K. The observed solvent effects on reactivity are analysed in terms of initial state and transition state contributions, and the pattern established compared with patterns for methanol- and acetone-water mixtures. In all cases the hydrophobic periphery of the ligand is dominant. Rate constants are also reported for racemisation and dissociation of the analogous complex [Fe(sb)3]2+, where sb = the Schiff base from 2-benzoylpyridine andp-toluidine, in aqueous methanol.

Initial state and transition state contributions to solvent effects on rate constants for racemisation and dissociation of the [Fe(phen) 3 ] 2+ cation in DMSO-water mixtures

We report rate constants for reactions of tris(2,2′-bi(4H-5,6-dihydrothiazine)iron(II), Fe(btz)
 3
 2+
 , in acid solution, with hydroxide, with peroxodisulphate, with silver(I), with mercury(II), and with cadmium(II), in aqueous solution, and for the reaction of Fe(btz)3

 3
 2+
 with 1,10-phenanthroline and with 2,2′-bipyridyl in aqueous methanol Mechanisms and relative reactivities are discussed

Kinetics of reactions of the tris(2,2′-bi(4H-5,6-dihydrothiazine))iron(II) cation

Activation enthalpies have been determined for aquation of thetris(1,10-phenanthroline)iron(II) cation and for its reaction with hydroxide in 20, 40, and 80% (by volume) methanol, and for its reaction with cyanide in 20 and 40% methanol. Enthalpies of solution of the chloride salt of this cation, and of potassium cyanide, have been determined in the water-methanol mixtures. From these results and ancillary thermodynamic data we have dissected the variations of activation enthalpies for these three substitution reactions with solvent composition into their initial state and transition state components. In the water-rich mixtures (0 to 40% methanol) initial and transition state solvation effects are similar and dominated by the ligands, but in the reaction with hydroxide in 80% methanol there is a striking difference between initial and transition state behaviour.

Initial state and transition state contributions to solvent effects on activation enthalpies for substitution reactions of tris(1,10-phenanthroline)iron(II) in aqueous methanol

Timothy Digman

Papers

Reactivity and solvatochromism of 2,2′-bi(4H-5,6-dihydrothiazine)-tetracarbonylmolybdenum(O)

Initial state and transition state contributions to solvent effects on rate constants for racemisation and dissociation of the [Fe(phen) 3 ] 2+ cation in DMSO-water mixtures

Kinetics of reactions of the tris(2,2′-bi(4H-5,6-dihydrothiazine))iron(II) cation

Initial state and transition state contributions to solvent effects on activation enthalpies for substitution reactions of tris(1,10-phenanthroline)iron(II) in aqueous methanol