Ligand-centered oxidation-reduction

Protonation of dinitrogen or hydrazido(2-) ligands to yield ammonia [reactions (44)—(50)] is not coupled to electron transfer from an external reductant. Electrons for (new) N—H electron pair bonds must therefore come from the metal as is the case in reaction (46), or from ligand-centered oxidation or disproportionation reactions, as appears to be the case in reaction (49). 

Reactions of coordination complexes can be conveniently divided into substitution reactions at the metal center, oxidation-reduction reactions, and reactions of the ligands that do not change the attachments to the metal center. Reactions that include more elaborate rearrangements of ligand structures are more often observed in organometallic compounds description of these reactions is given in Chapter 14. 

Al, Ga polymers. Variable-temperature conductivity measurements also indicate semi-conducting behaviour with of 0.1 eV. The /i-cyanophthalocyaninato polymers of Mn and Cr exhibit lower conductivity in the range of 10 to 10 S cm than the corresponding cobalt or iron compounds [87, 104,114]. The large conductivity of //-cyano cobalt and iron compounds indicates that the metal atom plays a role in the conduction process. EPR measurements indicate ligand-centered oxidation and the presence of a penta coordinated metal that metal-centered reduction has occurred [108]. 

Complexes (690) undergo two one-electron reversible reductions and two oxidations, all of which appeared ligand centered. Thus, these ligands behave electrochemically much like... 

Larger dendrimers based on a Ru(bpy)2+ core and containing up to 54 peripheral methylester units (12) have recently been obtained [29a]. Both the metal-centered oxidation and ligand-centered reduction processes become less reversible on increasing dendrimer size [29b]. 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

It is remarkable that the oxidized states of the cytochromes cdi from P pantotrophus and P. aeruginosa have different structures. It is not clear at present whether one of these structures is superior for catalyzing nitrite reduction. Certainly in the P. pantotrophus enzyme the ligand switching at both ligand centers upon changing oxidation state is... 

The CVs of alkynylbipyridyl-ruthenium complexes, such as [Ru(bpy)2(5-HC= C-bpy)] " and the related terpy complex, [Ru(terpy)(4-HC=C-terpy)] +, contain reversible metal-centered oxidation waves and several ligand-dependent reduction... 

Combination and rearrangement of Eqs. (16) and (23) lead to expression (24) that correlates linearly the ligand-centered reduction potential with the metal centered oxidation potential (d is the denticity of the reducible LL ligand). The slope of this line is Yl/Ym, and the intercept is a constant for the particular reducible ligand [66]. 

Relationships between redox potentials and the energy hv) of a metal-to-ligand charge transfer (M LCT) band have been well documented and expressed, for complexes [M(LL)WXYZ], in a simplified way, by Eq. (25) in which C is a constant and A (Redox) is given by Eq. (26), that is, the difference between the metal centered oxidation potential and the ligand centered reduction potential [67]. 

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