Metal carbonyls oxidation potential

A different application of eel is illustrated by the alternating current electrolysis of transition metal carbonyl complexes. Figure 6 shows the cycling of a complex A between oxidized and reduced forms. Use of alternating current electrolysis enables chemical use to be made of eel. For example, reaction (42) is known to be achievable by conventional photochemical means and can also be carried out by applying an alternating potential in an electrochemical cell.91... 

The hydroacylation of olefins with aldehydes is one of the most promising transformations using a transition metal-catalyzed C-H bond activation process [1-4]. It is, furthermore, a potentially environmentally-friendly reaction because the resulting ketones are made from the whole atoms of reactants (aldehydes and olefins), i.e. it is atom-economic [5]. A key intermediate in hydroacylation is a acyl metal hydride generated from the oxidative addition of a transition metal into the C-H bond of the aldehyde. This intermediate can undergo the hydrometalation ofthe olefin followed by reductive elimination to give a ketone or the undesired decarbonyla-tion, driven by the stability of a metal carbonyl complex as outlined in Scheme 1. 

The carbonylation oxidative polycondensation of bisphenol, 2,2-bis(4-hydroxyphenyl)propane, with transition metal-based catalysts, which yields the respective aromatic polycarbonate, is of high potential interest [6] ... 

A potentially important probe of the bonding in alkynyl complexes, and, in particular, the extent of M—> CCR 7r-backbonding, is to determine how the M—C=C—R fragment is structurally perturbed by the oxidation or reduction of the complex. The M—C=0 bond lengths of metal-carbonyl complexes are extremely sensitive to oxidation state, with M-C bonds lengthening and C=0 bonds contracting as the oxidation state of the metal increases (and, concomitantly, M - CO rr-backbonding decreases) (154). 

Table 3. Vertical ionization potentials (IP) and oxidation (peak) potentials ( ox) of metal carbonyls.

The ionization and oxidation (peak) potentials [53] of metal carbonyls listed in Table 3 establish their mild donicity despite their (formally) neutral oxidation state. However, partial replacement of carbonyls by stronger donor ligands (such as phosphine, sulfide, etc.) effects an incremental increase in their reducing properties [54], Such a fine-tuning of the oxidation potentials of structurally similar organometallic donors is ideal for studies of the correlation between rate constants and the electron-transfer driving force (see Section 2.5). 

Similarly, the mild reducing power of metal carbonyls can be effectively enhanced by conversion of the neutral complex to the corresponding carbonyhnetallate. For example, hexacarbonylvanadate and pentacarbonyhnanganate exhibit oxidation potentials about 1 V lower than those of the isoelectronic hexacarbonylchromium and pentacarbonyliron complexes, respectively (see Table 6). Similar trends apply to neutral and negatively charged half-sandwich complexes such as the isoelectronic CpCr(CO)3 and CpMn(CO)3 in Table 6. 

The 2e oxidation of the metal-carbonyl anion to the dimer (2M — M2 -1- 2e ) is evaluated as a combination of the oxidation potential of the anion and the metal-metal bond strength [44]. 

These complexes exhibit in dichloromethane solution a first ferrocene-based oxidation with features of chemical reversibility, followed by a second irreversible oxidation, which is thought to be centered on the metal-carbonyl fragment. A poorly reproducible oxidation of the Au moiety is also observable [49]. The relevant redox potentials are summarized in Table 7-35. With respect to the oxidation of free l,T-bis(diphenylphosphino)ferrocene E° = - -0.66 V) [49], the multimetallic com-plexation makes oxidation of the ferrocene fragment significantly more difficult (by about 0.4 V). 

The activation by cr-n interacation is also applicable for the acyl metal compounds such as acylsilanes. The oxidation potentials of acylsilanes are less positive than those of aldehydes and ketones because the nonbonding p orbital of the carbonyl oxygen atom... 

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