Between Two Metal Centers

The pioneering studies of Taube and his co-workers established remote attack by Cr on an extended organic ligand attached to M = Co(III), Cr(III) or Ru(III)  

The result made it possible to begin to answer the importrmt question as to how electron transfer from one redox center to another occurred. Was it (i) by the passage of electrons to 

This is called a chemical, radical or stepwise mechanism. Or was it (ii) by the action of the bridging group to increase the probability of electron transfer by tunneling, termed resonance transfer 56,9i 

The chemical mechanism was supported for M = Co and Cr, whereas a resonance transfer was favored by Ru(III) and these differences were rationalized. As important as these 

Chart 5.1 Bridged Ligand Systems Within Which Electron Transfer Occurs 

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The inner-sphere mechanism is restricted to those complexes containing at least one ligand which can bridge between two metal centers. The commonest examples of such ligands are the halides, hydroxy or oxo groups, amido groups, thiocyanate... 

A number of dialkyldithiophosphate zinc complexes were prepared and characterized in liquid and solid states, and as surface complexes on sphalerite. 31P NMR was used to characterize the coordination modes, whether terminal chelating or bridging between two metal centers. Correlations were made between data for the structurally characterized O.O -dicyclohexyldithiophos-phate zinc and the NMR of the surface complexes demonstrating bridging coordination between two neighboring zinc atoms on the synthetic sphalerite.568... 

A somewhat related problem is the nature of the bridged carbonyl group between two metal centers. Obvious correlations with organic ketonic behavior in general provide difficulties. In general nucleophilic attack by OR- (R = H or Me) does appear to occur at the carbon center. For R = Me, stable M- C02R complexes may often be isolated, but for R = H, transfer of hydrogen to the metal with elimination of C02 occurs readily, to yield the hydridocarbonyl. 

The benzene molecule is attached as a phenyl group, between two metal centers (structure I), and as a "benzene four-electron donor, three-center coordinating group in structures II, VI, and X. The C-C distance in all these molecules is 1.42 A, and the dihedral angle between the planes containing the osmium atoms and the mean plane of the benzyne in the three complexes is remarkably constant at 69 3°. 

A general summary of the chemistry of H4Os4(CO)12 is given in Scheme 10 (159, 166). Substitution reactions with phosphine, phosphites, or pyridine have been observed. An addition reaction occurs with I2 to yield the anionic species [H3Os4(CO)12I] The X-ray structure of this anion has established that the iodide is bridging between two metal centers, to yield an Os4 "butterfly configuration in which one of the metal- metal bonds of the initial Os4 tetrahedron has been broken (Os-Os = 3.817 A) (Fig. 38). Interestingly, if the two Os-1... 

Bielawski et al. have developed Janus-head dicarbene ligands which are able to act as a bridge between two metal centers, thereby leading to dinuclear complexes of type 96 [58-60] (Fig. 32). More recently homonuclear bimetallic ruthenium(II) and iron(II) complexes 97 have been synthesized. It was hoped that the dicarbene ligand would interconnect the redox-active metal centers, but the... 

Electron transfer between two metal centers sharing a ligand or atom in their respective coordination shells. See Metal Ion Coordination (Basic Chemical Behavior)... 

The methylene isomer 15 eliminates carbon monoxide when heated at 110°C (toluene, xylene 24 h), and yet another carbon-metal hydrogen shift takes place (75). The methylidyne species 16formed in this irreversible reaction has been characterized by its NMR spectra [S(CH) 9.36, quartet (76) 8(CH) 118.4 (CD3)2CO (77)] which implies a structure analogous to the previously known ethylidyne derivative (/i.-H)3(/i.-CCH3)-Os3(CO)io (138). NMR data obtained for partially deuterated samples of the methyl complex 14 have yielded evidence that the methyl ligand occupies an unsymmetrical bridging position between two metal centers, a coordination mode which implies significant C H Os interaction... 

Fig. 6. Relationship between A value and the distance R between two metal centers in structures 16a-d...

When C02 is bound between two metal centers, the ones that lose C02 most readily are the x2-r 2 and x2-r 3 complexes, in which one or two of the carboxylate oxygen atoms is (are) bound to a main group atom. These complexes are often intermediates in photochemical and electrochemical reduction reactions of C02 to CO. 

This finding can be explained in terms of heme-heme interaction, a phenomenon first suggested by Nicholls (19) and now well established by other observations (2). Wilson et al. (20) have suggested that certain ligands induce a cooperativity between two metal centers, similar to that in laccase so that these function as a 2-e unit. 

Inner-sphere (electron transfer) — is, historically, an - electron transfer between two metal centers sharing a ligand or atom in their respective coordination shells. The term was then extended to any case in which the interaction energy between the donor and acceptor centers in the -> transition state is significant (>20 kj mol-1). See also -> Marcus theory. 

Metal to metal charge transfer (MMCT) transition An electronic transition of a bi-or poly-nuclear metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer between two metal centers has occurred. 

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