Ligand 7r-acceptor

Dialkylzinc compounds have a tendency to form colored, and sometimes paramagnetic, adducts with 7r-acceptor ligands. The potentially bridging nitrogen donor pyrazine (and 4,4 -bipyridine) formed dinuclear complexes, Scheme 31, while the chelating 2,2 - bipyridine formed mononuclear adducts with diisopropylzinc.78 In the bis(diisopropylzinc)-pyrazine adduct 33, the presence of an unpaired electron was detected, but the resolution of the ESR spectra was insufficient for an unambiguous structural assignment. 

Presumably these transformations begin with one or more simple ligand replacement steps but these early intermediates are too unstable or reactive to be detected because of the action of the 7r-acceptor ligands in draining off electrons from the M-M tt and 6 bonds. 

A large proportion of mononuclear transition-metal complexes involving 7r-acceptor ligands can be considered to possess the 18 electrons of an inert-gas shell configuration. Justification for this can read-... 

The operation of the cis effect of the porphyrin may be explained according to Case D (Fig. 1) the more basic the porphinato ligand, the more electron density it induces at the metal ion, and the more will the d -electrons at the metal tend to flow away to the 7r-acceptor ligand X, reinforcing the M—X bond (90). An alternative ra-... 

This predominant effect of the 7r-acceptor ligand is also graphically expressed for the correlation of the wavenumber of the a-band and the Osn/Osnl redox potential in Fig. 9, which demonstrates the increase in 7r-acceptor capacity for the ligands ac-... 

To summarize, the special bonding characteristics of 7r-acceptor ligands in organotransition metal compounds enable these complexes to coordinate small molecules such as ethylene, CO, and H2 and also provide an electronic buffer system to facilitate changes of metal oxidation state and coordination... 

Firstly, the metal is invariably found in a relatively low oxidation state, typically in the range of 2+ to 3+ for species without 7r-acceptor ligands. When ligands such as carbon monoxide are present, the removal of electron density from the metal that occurs alters this criterion such that lower oxidation states such as 0+ and 1 + are common. 

The ortho buttressing of the TMP ligand was found to be essential for catalytic activity. Thus, the tetra(p-tolyl) porphyrin Ru(TTP)(THF)2, which is known to form an inactive p-oxodiruthenium(IV) complex upon oxygenation, was inactive as an oxygenation catalyst, as were RuCO(TMP)L and Ru[P(OMe)3]2(TMP). This means that the reaction medium must not contain 7r-acceptor ligands which prevent an autoxidation of Ru(II) porphyrins. 

The radial charge redistribution caused by MLCT excitation outside the central atom can be effective in its oxidation and reduction of the ligand or even an external solvent molecule. These reaction modes are observed in complexes of metals at their low oxidation numbers (eg Cu1, Au1, Fe11, Ru11, MoIV) with relatively good 7r-acceptor ligands, eg CN, NCS, NO+, 0, pyridine, bipyridine, 1,10-phenanthroline, methylene blue) [48,53,79]. Often the final product is really not the oxidized, but only the substituted complex, eg ... 

Chelate coordination-assisted SET reactivity as described in Eq. (12) may be invoked in some remarkable insertion reactions of alkylaluminium porphyrins [139] and are not restricted to the main group organometallics. Photoreactions of low-spin d systems with a filled fag sub-shell such as Co(III) [140] or Pt(IV) [141] with chelating 7r-acceptor ligands also involve CT absorptions and apparent radical pair formation (spin-trapping, CIDNP [141]) before typical followup reactions such as C-alkylation [38,95,140] occur. 

Conventionally, carbene complexes are grouped into two classes Fischer [2] and Schrock [3] carbenes. Fischer carbenes may be identified by their 7r-donor substituents on the carbene carbon and having 7r-acceptor ligands bonded to a low-oxidation state, late-transition metal. These ligands may be thought of as singlet carbenes that donate a... 

Spin-allowed LF transitions are always Interconfigurational. In complexes of 7r-acceptor ligands these transitions also involve promotion of an electron from orbitals which are x-bondlng to orbitals which are o-antibonding with respect to the M-L bonds. 

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