Two-electron donors

Exopolyhedral Metallacarboranes. Many metaHacarboranes are known that exhibit exopolyhedral bonding to metals. Most commonly metals are bound via M—H—B interactions in which the B—H group can be regarded as a two-electron donor to the metal center. In other cases, M—B,... 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

This has a folded structure (Figure 2.27) similar to that of rhodium carbonyl chloride (Figure 2.18) with ethene acting as a two-electron donor, but ethene is more weakly held and readily displaced by CO and certain alkenes (e.g. cycloocta-1,5-diene). 

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],... 

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). 

When using the eighteen electron rule, we need to remember that square-planar complexes of centers are associated with a 16 electron configuration in the valence shell. If each ligand in a square-planar complex of a metal ion is a two-electron donor, the 16 electron configuration is a natural consequence. The interconversion of 16-electron and 18-electron complexes is the basis for the mode of action of many organometallic catalysts. One of the key steps is the reaction of a 16 electron complex (which is coordinatively unsaturated) with a two electron donor substrate to give an 18-electron complex. 

Initial work was carried out with 3,9-bis(methylene-2,4,8,10-tetraoxaspiro[5,5] undecane) where R = H (11). However, this monomer contains two electron donor alkoxy groups on one double bond which is thus highly susceptible to a cationic polymerization. For this reason, the monomer is extremely difficult to handle and cannot be analyzed by gas chromatography since it does not survive passage through the column. It is prepared by the dehydrohalogen-ation reaction of the reaction product of pentaerythritol and chloro-acetaldehyde,... 

Formal valence state of iron L is a two-electron donor ligand Cp = 7i-cyclopentadienyl... 

Complexes shown in Figure 7 undergo facile bridge-splitting reactions with a variety of two-electron donor ligands to give mononuclear species.145... 

Fds with conventional [Fe2-S2] clusters can undergo a one-electron transfer to a deeply valence-trapped FemFen species. For proteins of known structure (and presumably others) one iron atom is closer to the surface (by about 0.5 nm) and it has been established that the added electron resides on that atom. No instances are known where an [Fe2-S2] centre acts as a physiological two-electron donor or acceptor. In addition to the conventional [Fe2-S2] ferredoxins, the electron-transfer chains of mitochondria and photosynthetic bacteria contain Rieske proteins which have a cluster with the composition [(Cys.S)2FeS2Fe(N.His)2], in which the two imidazole groups are bound to the same iron atom (Figure 2.9). This atom is the site... 

Fig. 20.4. H-bond acceptor patterns observed two electron donor groups with a spatial in P-gp substrates. Type I units patterns separation of 4.6 + 0.6 A. A denotes a informed by electron donor pairs with a spatial bonding acceptor group (electron donor...

The presence of a two-electron donor ligand in the organometallic substrate that can be readily displaced by the CRR fragment of the diazo precursor is required. Alternatively, a coordinatively unsaturated species may be a suitable reactant. 

Such characteristics led to the proposal (3,8) that the mechanism for the fragmentation pathway must involve the formation of a reactive intermediate, an isomer of RujCCO)] capable of first order return to the initial cluster or of capture by a two electron donor. Scheme 1 illustrates the proposed mechanism for photofragmentation. 

A possible formulation for I is illustrated below. This could be formed by the heterolytic cleavage of a Ru-Ru bond an corresponding movement of a carbonyl from a terminal site to a bridging one to maintain the charge neutrality of both Ru atoms. The result would be to leave one ruthenium atom electron deficient (a 16 electron species) and capable of coordinating a two electron donor to give another intermediate I. ... 

It is interesting to note that the photosubstitution intermediate 11 appears to be significantly more selective toward reaction with various two electron donor substrates than is the photofragmentation intermediate 1. One speculative rationalization of this is that the Ru3(C0) intermediate has the opportunity to "delocalize" its unsaturation by having one CO bridge an edge of the metal triangle with concomitant formation of a multiple metal-metal bond. 

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