Dissociative Exchange Research Articles

Reactions of hydrated electrons (H 2O) n− with formic acid

The chemistry of gas phase hydrated electrons (H 2O) n − with formic acid is studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Black body infrared radiative dissociation and ligand exchange of water by formic acid molecules take place, accompanied by a small contribution of an intracluster redox reaction. During incorporation of the first formic acid molecule, the electron may recombine with the acidic proton and evaporate as a hydrogen atom, leaving formate as new cluster core. At long time scales, all water molecules are exchanged against formic acid, and three formic acid molecules are sufficient to stabilize the electron. For m = 3 and 4, slow formation of (HCOOH) m−1 (HCOO −) is observed. This loss of hydrogen may be activated by collisions with HCOOH, resulting in [HCOOH + H] reaction products of unknown structure.

ACCESSORY PROTEINS FOR G PROTEINS: Partners in Signaling

Accessory proteins involved in signal processing through heterotrimeric G proteins are generally defined as proteins distinct from G protein-coupled receptor (GPCR), G protein, or classical effectors that regulate the strength/efficiency/specificity of signal transfer upon receptor activation or position these entities in the right microenvironment, contributing to the formation of a functional signal transduction complex. A flurry of recent studies have implicated an additional class of accessory proteins for this system that provide signal input to heterotrimeric G proteins in the absence of a cell surface receptor, serve as alternative binding partners for G protein subunits, provide unexpected modes of G protein regulation, and have introduced additional functional roles for G proteins. This group of accessory proteins includes the recently discovered Activators of G protein Signaling (AGS) proteins identified in a functional screen for receptor-independent activators of G protein signaling as well as several proteins identified in protein interaction screens and genetic screens in model organisms. These accessory proteins may influence GDP dissociation and nucleotide exchange at the G(alpha) subunit, alter subunit interactions within heterotrimeric G(alphabetagamma) independent of nucleotide exchange, or form complexes with G(alpha) or G(betagamma) independent of the typical G(alphabetagamma) heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell.

NMR Distinction of Single- and Multiple-Mode Binding of Small-Molecule Protein Ligands

Identifying and characterizing small-molecule inhibitors of protein-protein interactions is of high interest for drug discovery and for chemical genetics studies of biological pathways. Very often, initial hits or first-generation compounds have low micromolar dissociation constants and cause line broadening in NMR spectra. It is very important for subsequent structure-based compound optimization to know if this line broadening is caused by intermediate exchange of the dissociation kinetics only or in addition by multiple binding modes. Here, we present an approach of how to distinguish these two situations and demonstrate its experimental application. Two very similar small-molecule ligands of Bcl-xL are considered that cause both severe line broadening of interface residues. We show that one compound exhibits single-mode binding, and broadening is just due to dissociation kinetics in the intermediate exchange regime, and the line broadening can be overcome by providing excess ligand. In the other case, line broadening is due to dissociation kinetics and exchange between multiple bound conformations, and broadening cannot be overcome by providing excess ligand. The procedures used are very general and can also be applied to characterizing protein-protein and protein-nucleic acid interactions.

Synthesis and Reactivity of Molybdenum(IV) Complexes with Alkyl and Aryl Isocyanides

The synthesis and characterization of the bis(isocyanide) complexes (RNC)2Mo(NPh)(o-(Me3SiN)2C6H4) (2: 2a, R = tBuNC; 2b, R = 2,6-dimethylphenyl) and the subsequent reactivity of these complexes with excess isocyanide have been reported. An X-ray crystal structure of (RNC)2Mo(NPh)(o-(Me3SiN)2C6H4) (R = 2,6-dimethylphenyl) is reported. Treatment of 2a with excess tert-butyl isocyanide resulted in the formation of the tris(isocyanide) complex (tBuNC)3Mo(NPh)(o-(Me3SiN)2C6H4) (3). Complex 3 exists in solution in equilibrium with 2a. Using a two-site exchange mechanism the activation barrier for the dissociative exchange of tBuNC has been calculated. The X-ray crystal structure of (tBuNC)3Mo(NPh)(o-(Me3SiN)2C6H4) is reported. Computational studies (ONIOM) performed on 3 reveal that π conflicts in this complex results in the lengthening of the Mo−N(amido)cis bond relative to the Mo−N(amido)transbond. Treatment of 2b with excess 2,6-dimethylphenyl isocyanide results in the slow insertion of the isocyanide ligands into the Mo−N bond of the chelating diamide ligand, resulting in a novel chelating iminocarbamoyl bis(isocyanide) complex, 4. An X-ray crystal structure of 4 is reported.

Disilver(I) Rectangular-Shaped Metallacycles: X-ray Crystal Structure and Dynamic Behavior in Solution

Reaction of the ditopic semirigid ligand 1,2-bis(imidazolylmethyl)benzene (1,2-bImb) or the flexible ligand 1,4-bis(2-benzimidazolyl)butane (C4BIm) with AgX (X = ClO4-, BF4-, CF3CO2-) afforded five new complexes, namely, [Ag2(1,2-bImb)2](ClO4)2 (1), [Ag2(1,2-bImb)2](BF4)2 (2), [Ag2(1,2-bImb)2](CF3CO2)2.2CH3OH (3.2CH3OH), [Ag2(C4BIm)2](ClO4)2.2DMF (4.2DMF), and [Ag2(C4BIm)2](CF3CO2)2.2H2O (5.2H2O), all of which contain a centrosymmetric, rectangular-shaped cationic disilver(I) metallacycle [Ag2(L)2]2+. In 1-3, a pair of 1,2-bImb ligands takes on the syn conformation to connect two Ag(I) ions to give a compressed rectangle with a transannular Ag...Ag separation of 3.27-3.36 angstroms, whereas in 4 and 5, the pair of planar C4BIm ligands acts in the cis conformation to connect two Ag(I) ions to yield a normal rectangle with a transannular Ag...Ag separation of 7.67-7.91 angstroms. The anions form Ag...O or Ag...F weak interactions in 1-3 and O-H...O or N-H...O hydrogen bonds in 4 and 5 in crystal packing but exhibit no significant influence on the formation of the disilver(I) macrocycles. The solution structure and dynamic behavior of the complexes studied by electrospray ionization mass spectrometry, 1H NMR, and variable-temperature NMR indicated that the dynamic equilibrium between the [Ag2(L)2]2+ cation and the open-ring oligomers or other potential species occurs via solvent-assisted dissociative exchange. The metal-ligand exchange barrier was estimated to be 54.5 kJ mol(-1).

Synergistic Dimetallic Effects in Propargylic Substitution Reaction Catalyzed by Thiolate-Bridged Diruthenium Complex

The origin of unique catalytic activity of a thiolate-bridged diruthenium complex in nucleophilic substitution reactions of propargylic alcohols, which features a diruthenium-allenylidene complex as a key intermediate, was studied with the aid of density functional calculations (B3LYP). Comparison of mono- and diruthenium systems has shown that the rigid but reasonably flexible Ru-Ru core structure plays a critical role in the catalyst turnover step (i.e., dissociative ligand exchange of the product pi-complex with the starting propargyl alcohol that goes through a coordinatively unsaturated Ru complex). In the diruthenium system, the energy loss due to coordinative unsaturation can be compensated by reinforcement of the Ru-Ru bond, while such an effect is unavailable in the monoruthenium counterpart. Weaker back-donation ability of the diruthenium complex is also advantageous for dissociation of the pi-complex. Thus, ligand exchange takes place smoothly in the diruthenium system to regenerate the reactive species, while the monoruthenium reaction stops at a dead-end Ru product pi-complex. The present studies have also shown the important role of protic molecules (e.g., MeOH) that mediate smooth proton transfer in the propargyl alcohol-allenylidene transformation.

Effect of Octabromo Substitution on the Coordination Properties of Manganese(III) Octaphenyltetraazaporphyrin

Comparative studies of the kinetics of formation of manganese(III) complexes with octaphenyltetraazaporphyrin and its octabromo derivative, of their dissociation and axial ligand exchange were carried out. The effect of octabromo substitution on the coordination properties of the compounds is determined by the -I effect of bromine atoms. The different effects on the kinetic parameters of the reactions [increase in the rate and significant decrease in the activation energy upon complex formation, slowing down of dissociation, and acceleration of acido ligand exchange with nitrido(tetraphenyporphyrin)manganese(V) are caused by the involvement of different reaction centers in the limiting stages (different mechanisms of the processes).

Use of Phosphorus Ligand NMR Probes To Investigate Electronic and Second-Sphere Solvent Effects in Ligand Substitution Reactions at Manganese(II) and Manganese(III)

Manganese/ligand association dynamics were studied using a series of structurally related anionic phosphorus ester ligand probes [CH(3)OP(O)(X)(Y)(-), where X = CH(3)O, CH(3)CH(2), or H and Y = O, S, or BH(3)]. Reactions of the probe ions with Mn(H(2)O)(6)(2+) and a manganese(III) porphyrin (Mn(III)TMPyP(5+)) were studied in aqueous solution by paramagnetic (31)P NMR line-broadening techniques. A satisfactory linear free energy relationship for reactions of the probe ions with Mn(H(2)O)(6)(2+) and Mn(III)TMPyP(5+) required consideration of both the basicity and solvent affinity of the probe ligands: log(k(app)) = log(k(0)) + alpha pK(a) + beta log(K(ext)), where k(0), alpha, and beta are metal complex dependent parameters and pK(a) and K(ext) represent the measured Bronsted acidity and water/n-butanol extraction constant for the probe anions, respectively. Reactions of Mn(H(2)O)(6)(2+) were relatively insensitive to changes in ligand basicity (alpha = -0.04) and favored the more hydrophilic anions (beta = -0.54). These observations are consistent with a dissociative ligand exchange mechanism wherein the outer-sphere complex is stabilized by hydrogen bonding between Mn(H(2)O)(6)(2+) and the incoming ligand. In contrast, reactions with Mn(III)TMPyP(5+) are accelerated by decreases in both the basicity (alpha = -0.43) and the hydrophilicity (beta = +0.97) of the probe. We conclude that reactions of Mn(III)TMPyP(5+) are also dissociative but that the aromatic groups of the porphyrin provide a hydrophobic environment surrounding the ligand binding site in Mn(III)TMPyP(5+). Thus, the probe/water solvent interactions must be significantly weakened in order to form the outer-sphere complex that leads to ligand substitution. This work demonstrates the utility of phosphorus relaxation enhancement (PhoRE) techniques for characterizing the second coordination sphere environment of metal complexes leading to ligation and will allow comparison of the second coordination spheres of Mn(H(2)O)(6)(2+) and Mn(III)TMPyP(5+) to those of other metal complexes.

Water‐Exchange Mechanism for Zinc(II), Cadmium(II), and Mercury(II) Ions in Water as Studied by Umbrella‐Sampling Molecular‐Dynamics Simulations

AbstractUmbrella‐sampling molecular‐dynamics simulations were performed to investigate the water‐exchange reactions of zinc(II), cadmium(II), and mercury(II) ions in aqueous solution. The dissociation of a coordinating water molecule to the MO distance at 3.34, 3.16, and 3.26 Å for ZnII, CdII, and HgII, respectively, leads the system to a transition state. For ZnII, the first hydration shell is occupied by five spectator water molecules in the transition state, indicating that the water‐exchange reaction proceeds via a dissociative mode of activation. In contrast, the number of spectator water molecules of 5.85 and 5.95 for CdII and HgII, respectively, suggests an associative exchange for these larger metal ions. The average MO distance of the spectator molecules is shortened by 0.06 Å for the dissociative exchange of ZnII, while it is elongated by 0.04 and 0.03 Å for CdII and HgII, respectively. The water‐exchange rate constants of 4.1×108, 6.8×108, and 1.8×109 s−1 are estimated for ZnII, CdII, and HgII, respectively, at 298 K in terms of the transition‐state theory based on the assumption of a transmission coefficient of unity.

Modelling of H/D exchange over Pd

Abstract The isotopic exchange between hydrogen and deuterium was studied over a Pd/alumina catalyst. A model was proposed, including the adsorption and dissociation of hydrogen and deuterium. An adsorption-assisted desorption mechanism was also included and turned out to play a minor role in the H 2 /D 2 exchange but have to be considered in the reaction between deuterium and water. A comparison between the experimental data and the simulation revealed that the model could describe the observed dissociation and isotope exchange reaction.

Kinetics and modeling of H 2/D 2 exchange over Ag/Al 2O 3

An alumina supported silver catalyst was studied with a transient step-response technique for the reduction of NO and N 2O with H 2. Dissociation of hydrogen and deuterium was observed on the Ag/alumina catalyst and a kinetic model was proposed for the isotopic exchange between H 2 and D 2. The proposed model describes the observed dissociation and isotope exchange reaction sufficiently well.

Cationic Platinum(II) Carboxylato Complexes Are Competent in Catalytic Arene C−H Activation under Mild Conditions

A combination of ESI-MS and kinetic studies establishes that the PtII-catalyzed H/D exchange between benzene and acetic acid-d4 proceeds with high turnover number in acetic acid solution. For the catalytic cycle, the resting state is identified as the cationic (diimine)PtII(acetato)(acetic acid) complex; the turnover-limiting transition state is the dissociative ligand exchange of benzene for acetic acid. The interconversion of η2- and η1-acetato is surprisingly facile, leading to an unexpectedly fast H/D exchange reaction.

Compositional regulation of poly(lysine- g-(lactide- b-ethylene glycol))–DNA complexation and stability

The search for an effective nonviral gene therapy vector has revealed several significant hurdles, such as transient expression and cytotoxicity, that impede the success of these systems. A terpolymer of poly(lysine- g-(lactide- b-ethylene glycol)) [pK- g-p(LL- b-EG)] has been developed, which is capable of stably packaging DNA with significantly less polycation than unmodified polylysine (pK) systems. A comparison of unmodified pK (DP=14, M w=2930) to pK grafted with p(LL- b-EG) ( M w tot=7500) showed that the minimum amine/phosphate ratio (N:P min) needed for complete DNA condensation was reduced by 50%. However, when the molecular weight of pK was reduced (DP=4, M w=838), there was evidence of terpolymer interference with DNA condensation. Increasing the number of grafted p(LL- b-EG) chains produced a similar result of incomplete DNA condensation. All terpolymer formulations produced complexes with DNA that had greater resistance to salt-induced dissociation and short-term exchange with excess DNA. Terpolymer–DNA complexes exhibited approximately zero-order plasmid release profiles over a period of 6 weeks. The rate of release was dependent on the complex N:P ratio as well as the molecular weights of pLL and pK. The ability to use terpolymer composition to control complex stability and controlled release can provide a means for system optimization for sustained expression profiles of exogenous DNA.

Modeling water exchange on monomeric and dimeric Mn centers

Water exchange on Mn centers in proteins has been modeled with density functional theory using the B3LYP functional. The reaction barrier for dissociative water exchange on [MnIV(H2O)2(OH)4] is only 9.6 kcal mol−1, corresponding to a rate of 6×105 s−1. It has also been investigated how modifications of the model complex change the exchange rate. Three cases of water exchange on Mn dimers have been modeled. The reaction barrier for dissociative exchange of a terminal water ligand on [(H2O)2(OH)2MnIV(μ-O)2MnIV(H2O)2(OH)2] is 8.6 kcal mol−1, while the bridging oxo group exchange with a ring-opening mechanism has a barrier of 19.2 kcal mol−1. These results are intended for interpretations of measurements of water exchange for the oxygen evolving complex of photosystem II. Finally, a tautomerization mechanism for exchange of a terminal oxyl radical has been modeled for the synthetic O2 catalyst [(terpy)(H2O)MnIV(μ-O)2MnIV(O•)(terpy)]3+ (terpy=2,2′:6,2″-terpyridine). The calculated reaction barrier is 14.7 kcal mol−1.

Mechanism of copper(I)-catalyzed cyclopropanation: a DFT study calibrated with copper(I) alkene complexes

The complete catalytic cycle of copper-catalyzed cyclopropanation is investigated by density functional model calculations. The study focuses on obtaining accurate relative stabilities of the ligand–metal cores of all catalytic intermediates and transition states. The copper iminophosphanamide [H 2P(NH 2) 2κ 2 N]Cu serves as model fragment, since three copper catalyst key intermediates with an iminophosphanamide ligand have been isolated or detected. The electronic structure of the active intermediates, copper(I) carbenes of the d 10-ML 3 type, and their reactivity towards ethene and vinyl alcohol is investigated. The reliability of the computed data is confirmed by their comparison with experimental data of [ t-Bu 2P(NSiMe 3) 2κ 2 N]Cu alkene and alkyne complexes, using relative ligand binding strengths and alkene ligand rotation barriers. The uncharged nature of the model complexes minimizes solvation artifacts and thus renders the evaluation of associative and dissociative ligand exchange pathways possible. The associative alkene ligand exchange with diazoalkane to a κ N,κ O-diazoalkane complex, subsequent intramolecular rearrangement to a κ C-complex and N 2 extrusion are identified on the most probable pathway to the electrophilic copper(I) carbene complexes of the Fischer type. They are predicted to react with alkenes to labile cupracyclobutanes with a planar N 2Cu IIIC 2 core, which undergo facile reductive elimination of a cyclopropane derivative.

Rad54, a Swi2/Snf2-like Recombinational Repair Protein, Disassembles Rad51:dsDNA Filaments

Rad54 protein is a member of the Swi2/Snf2-like family of DNA-dependent/stimulated ATPases that dissociate and remodel protein complexes on dsDNA. Rad54 functions in the recombinational DNA repair (RAD52) pathway. Here we show that Rad54 protein dissociates Rad51 from nucleoprotein filaments formed on dsDNA. Addition of Rad54 protein overcomes inhibition of DNA strand exchange by Rad51 protein bound to substrate dsDNA. Species preference in the Rad51 dissociation and DNA strand exchange assays underlines the importance of specific Rad54-Rad51 protein interactions. Rad51 protein is unable to release dsDNA upon ATP hydrolysis, leaving it stuck on the heteroduplex DNA product after DNA strand exchange. We suggest that Rad54 protein is involved in the turnover of Rad51-dsDNA filaments.

Site-differentiated hexanuclear rhenium(III) cyanide clusters [Re(6)Se(8)(PEt(3))(n)()(CN)(6)(-)(n)()](n)()(-)(4) (n = 4, 5) and kinetics of solvate ligand exchange on the cubic [Re(6)Se(8)](2+) Core.

The site-differentiated, cyanide-substituted hexanuclear rhenium(III) selenide clusters cis- and trans-[Re(6)Se(8)(PEt(3))(4)(CN)(2)] and [Re(6)Se(8)(PEt(3))(5)(CN)](+) have been prepared from heterogeneous reactions of the corresponding iodo clusters with AgCN in refluxing chloroform. Isolated yields are 68%, 46%, and 64% for cis-[Re(6)Se(8)(PEt(3))(4)(CN)(2)], trans-[Re(6)Se(8)(PEt(3))(4)(CN)(2)], and [Re(6)Se(8)(PEt(3))(5)(CN)](+), respectively. The new compounds are air- and water-stable and are characterized by X-ray diffraction crystallography, (31)P NMR and IR spectroscopies, and FAB mass spectrometry. In related work, the solvent exchange rates of two site-differentiated monosolvate clusters, [Re(6)Se(8)(PEt(3))(5)(MeCN)](SbF(6))(2) and [Re(6)Se(8)(PEt(3))(5)(Me(2)SO)](SbF(6))(2), in neat solvents were measured by (1)H NMR. These clusters are substitutionally inert; k approximately 10(-)(5)-10(-)(6) s(-)(1) at 318 K. Activation parameters indicate a dissociative ligand exchange mechanism; DeltaH() values obtained from least-squares fitting of temperature-dependent kinetics data exceed RT by a factor of ca. 50 over the temperature range studied. These results demonstrate that the substitutional lability encountered in a previous study of cluster photophysics (Gray, T. G.; Rudzinski, C. M.; Nocera, D. G.; Holm, R. H. Inorg. Chem. 1999, 38, 5932) cannot result from ground-state thermal reactions.

Palladium(II) beta-agostic alkyl cations and alkyl ethylene complexes: investigation of polymer chain isomerization mechanisms.

A series of stable dialkyl complexes of Pd, (alpha-diimine)PdR2 (alpha-diimine = aryl-substituted diimine, R = n-Pr, n-Bu, i-Bu), have been prepared via Grignard alkylation of the corresponding (alpha-diimine)PdCl2 complexes. Protonation of these dialkyl species at low temperature results in loss of alkane and formation of cationic Pd beta-agostic alkyl complexes, which have been observed as intermediates in the polymerization of ethylene and propylene by these Pd catalysts. Studies of the structure and dynamic behavior of these alkyl complexes are presented, along with the results of trapping reactions of these species with ligands such as NCMe, CO, and C2H4. Trapping with ethylene results in formation of cationic alkyl ethylene complexes which model the catalyst resting state in these systems. These complexes have been used to obtain mechanistic details and kinetic parameters of several processes, including isomerization of the alkyl ethylene complexes, associative and dissociative exchange with free ethylene, and migratory insertion rates of both primary and secondary alkyl ethylene species. These studies indicate that the overall branching observed in polyethylenes produced by these Pd catalysts is governed both by the kinetics of migratory insertion and by the equilibria involving the alkyl ethylene complexes.

Mechanistic studies of nitric oxide reactions with water soluble iron(II), cobalt(II), and iron(III) porphyrin complexes in aqueous solutions: implications for biological activity.

The reactions of nitric oxide and carbon monoxide with water soluble iron and cobalt porphyrin complexes were investigated over the temperature range 298-318 K and the hydrostatic pressure range 0.1-250 MPa [porphyrin ligands: TPPS = tetra-meso-(4-sulfonatophenyl)porphinate and TMPS = tetra-meso-(sulfonatomesityl)porphinate]. Large and positive DeltaS(double dagger) and DeltaV(double dagger) values were observed for NO binding to and release from iron(III) complexes Fe(III)(TPPS) and Fe(III)(TMPS) consistent with a dissociative ligand exchange mechanism where the lability of coordinated water dominates the reactivity with NO. Small positive values for Delta and Delta for the fast reactions of NO with the iron(II) and cobalt(II) analogues (k(on) = 1.5 x 10(9) and 1.9 x 10(9) M(-1) s(-1) for Fe(II)(TPPS) and Co(II)(TPPS), respectively) indicate a mechanism dominated by diffusion processes in these cases. However, reaction of CO with the Fe(II) complexes (k(on) = 3.6 x 10(7) M(-1) s(-1) for Fe(II)(TPPS)) displays negative Delta and Delta values, consistent with a mechanism dominated by activation rather than diffusion terms. Measurements of NO dissociation rates from Fe(II)(TPPS)(NO) and Co(II)(TPPS)(NO) by trapping free NO gave k(off) values of 6.3 x 10(-4) s(-1) and 1.5 x 10(-4) s(-1). The respective M(II)(TPPS)(NO) formation constants calculated from k(on)/k(off) ratios were 2.4 x 10(12) and 1.3 x 10(13) M(-1), many orders of magnitude larger than that (1.1 x 10(3) M(-1)) for the reaction of Fe(III)(TPPS) with NO.

Electron transfer in a trinuclear oxo-centred mixed-valence iron complex, in solid and solution statesDedicated to Professor Dieter Sellmann on the occasion of his 60th birthday.

Complexes [FeIII2MIIOL3] (M = Fe, Co, Ni, Cu; 2, 3, 4, 5) have been synthesised in which L2− is a pentadentate ligand designed to coordinate all three metal atoms in the central cluster and to inhibit dissociation and solvent exchange processes. Crystal structures for 2, 4 and 5 show threefold symmetry, attributed to rotational disorder. Magnetisation data for 2 indicate strong superexchange between basis oxidation states Fe(3+, 3+, 2+). Comparisons of IR spectra across the series of complexes confirm the non-threefold symmetry of the mixed-valence cluster on the vibrational time scale, both in the solid state and in solution. Proton NMR spectra in solution at room temperature do not distinguish the three iron sites, suggesting that pseudo-rotation by thermal electron transfer also operates. Cyclic voltammetry and spectroelectrochemical measurements show that the mixed-valence iron complex 2 can be oxidised reversibly to give the tri-iron(III) complex [Fe3OL3]+ and reduced reversibly and quasireversibly to give respectively [Fe3OL3]− and tri-iron(II) [Fe3OL3]2−, E0 = 85, −635, −1230 mV (versus Fc+/0) in dichloromethane (T = 298 K, 0.1 M [n-Bu4N][PF6]). Mossbauer spectra of 2 indicate significant valence delocalisation even at low temperature (4.2 K) with estimated valences Fe(2.9+, 2.9+, 2.2+) in the solid state. At higher temperatures no lifetime broadening is observed but additional Mossbauer absorptions are consistent with increasing proportions of trimer molecules with greater delocalisation, i.e. Fe(2.75+, 2.75+, 2.5+). In frozen solution (THF) the spectra indicate increasing proportions of molecules fully valence-delocalised on the Mossbauer time scale. The data are accounted for with a model which places the complex at the Robin–Day class III/II borderline. It combines strong superexchange with significant double exchange even at the lowest temperatures, while at higher temperatures in solution complete valence delocalisation occurs through intramolecular electron transfer at rates intermediate between the IR and NMR time scales.