Ligands electron-donor

Ligands electron donors—usually negatively charged. 

In addition to inflow and outflow of Fe, we need expressions for the rate of reduction of Fe(III) (hydr)oxides and for the reoxidation of Fe by oxygen. The rate of photochemical Fe(II) formation depends on the light intensity I and the surface concentration of the specifically adsorbed ligand (electron donor) ... 

Imidazole is a ligand electron donor in the a and n sense. It is a much better electron donor than are most of the other nitrogen heterocycles. The majority of imidazoles are especially good a donors, and moderately good n donors. It has been recently shown that imidazole with an aldehyde group in 2-position, (2-CHOimH) is a very strong n acceptor, comparable to... 

There is more to tire Wilkinson hydrogenation mechanism tlian tire cycle itself a number of species in tire cycle are drained away by reaction to fomi species outside tire cycle. Thus, for example, PPh (Ph is phenyl) drains rhodium from tire cycle and tlius it inliibits tire catalytic reaction (slows it down). However, PPh plays anotlier, essential role—it is part of tire catalytically active species and, as an electron-donor ligand, it affects tire reactivities of tire intemiediates in tire cycle in such a way tliat tliey react rapidly and lead to catalysis. Thus, tliere is a tradeoff tliat implies an optimum ratio of PPh to Rli. 

Wlien a strong electron-donor ligand such as pyridine is added to tlie reaction mixture, it can bond so strongly to tlie Rli tliat it essentially drains off all tlie Rli and shuts down tlie cycle it is called a catalyst poison. A poison for many catalysts is CO it works as a physiological poison in essentially the same way as it works as a catalyst poison it bonds to tlie iron sites of haemoglobin in competition witli O. ... 

Polyisoprenes of 94—98% as-1,4 content were obtained with lanthanum, cerium, praseodymium, neodymium, and other rare-earth metal ions (eg, LnCl ) with trialkyl aluminum (R3AI) (34). Also, a NdCl 2THF(C2H3)3A1 catalyst has been used to prepare 95% <7j -l,4-polyisoprene (35). <7j -l,4-Polyisoprene of 98% as-1,4 and 2% 3,4 content was obtained with organoalurninum—lanthariide catalysts, NdCl where L is an electron-donor ligand such as ethyl alcohol or butyl alcohol, or a long-chain alcohol, and is 1 to 4 (36). 

Although isoxazoles are comparatively weak electron donors, complexes with numerous metal ions, notable metal(II) ions, have been reported. The ligands include isoxazole and its methyl, phenyl, amino and hydroxy derivatives. They are listed with references in Table 5. 

The nitrogen atom in the (almost) linear metal-thionitrosyl complexes is 5p-hybridized and the NS ligand behaves as a three-electron donor. The N-S bond distances in metal complexes vary between 1.45... 

A number of tertiary phosphine ligands have been synthesized that also contain an alkene linkage capable of coordinating to a metal. A good example of this kind of coordination is formed in the complex of (tri-o-vinyl-phenyl)phosphine (Figure 2.29) with each alkene acting as a two-electron donor, a noble gas configuration is achieved [67],... 

A good example is the excited state of the tris(bipyridine)ruthenium(2+) ion, Ru(bpy)5+. This species results from the transfer of an electron from the metal to a ligand. In the language of localized valences, it is a ruthenium(3+) ion, coordinated to two bipyridines and to one bipyridyl radical anion in other words, [Ru3+(bpy)2(bpy )]2+. This excited state is a powerful electron donor and acceptor.17 The following equations show an example of each quenching mode ... 

A benzannulation reaction yielding the naphthoquinone 61 could also be performed with the ruthenium carborane-stabilised carbene 60 and 1-hexyne [56] (Scheme 36). The ruthenium carbene unit can be regarded as an 18-electron fragment containing a formal Ru(II) centre coordinated to a dianionic six-electron-donor cobaltacarborane ligand. 

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

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