Electronic complex

The phosphido complex, U(PPP)4 [163823-64-9] (PPP = P(CH2CH2P(CH3)2), was prepared and fully characterized (216). This complex was one of the first actinide complexes containing exclusively metal-phosphoms bonds. The x-ray stmctural analysis iadicated a distorted bicapped triganol prism with 3—3-electron donor phosphides and 1—1-electron phosphide, suggesting a formally 24-electron complex. Similar to the amido system, this phosphido compound is also reactive toward iasertion reactions, especially with CO (216). 

For trinuclear cluster complexes, open (chain) or closed (cycHc) stmctures are possible. Which cluster depends for the most part on the number of valence electrons, 50 in the former and 48 in the latter. The 48-valence electron complex Os2(CO)22 is observed in the cycHc stmcture (7). The molecule possesses a triangular arrangement of osmium atoms with four terminal CO ligands coordinated in a i j -octahedral array about each osmium atom. The molecule Ru (00) 2 is also cycHc and is isomorphous with the osmium analogue. 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

The crucial experiment suggesting that the H2 molecule might act as a dihapto ligand to transition metals was the dramatic observation that toluene solutions of the deep purple coordinatively unsaturated 16-electron complexes [Mo(CO)3(PCy3)2] and [W(CO)3-(PCy3)2l (where Cy = cyclohexyl) react readily and cleanly with Ha (I atm) at low temperatures to precipitate yellow crystals of [M(CO)3H2(PCy3)2] in 85-95% yield. The... 

There are two general routes to complexes. The first involves direct addition of molecular Ht either to an unoccupied coordination site in a 16-electron complex (as above) or by displacement of a ligand such as CO, Cl, H2O in the coordination sphere of an 18-electron complex in this latter case ultraviolet irradiation may be required to assist in the... 

A remarkable transformation of [(> -Ph2BpZ2)Mo(CO)2(i -pentadienyl)] (90OM1862) is the transformation with phosphines or phosphites of this 16-valence-electron species into 18-electron complexes hydrolysis leads to profound changes in the coordination sphere yielding 60. 

Its structure shows the trans-influence of hydride and pronounced distortion from square planar geometry (H—Rh-P 70.7°) owing to steric crowding. (RhH(PPh3)3 is rather less distorted (H-Rh-P 75.8°) [118a], This 16-electron complex shows no tendency to add an extra molecule of phosphine, unlike the less hindered RhH(PEt3)3. It is, however, an active... 

Van der Schaaf et al. described a synthesis of the 14-electron complex [RuHCl(PPr13)2] (32) from [RuCl2(COD)]A.,PPr31,isopropanol,and abase.Compound 32 is a suitable precursor for ruthenium carbene complex 33, as outlined in Scheme 10. Although 32 was isolated and structurally characterized, it may also be generated in situ for the preparation of the carbene complex 33 [18]. 

In section 2.5 we have examined the effect of promoters and poisons on the chemisorption of some key reactants on catalyst surfaces.We saw that despite the individual geometric and electronic complexities of each system there are some simple rules, presented at the beginning of section 2.5 which are always obeyed. These rules enable us to make some predictions on the effect of electropositive or electronegative promoters on the coverage of catalytic reactants during a catalytic reaction. 

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. 

The palladium(O) complex undergoes first an oxydative addition of the aryl halide. Then a substitution reaction of the halide anion by the amine occurs at the metal. The resulting amino-complex would lose the imine with simultaneous formation of an hydropalladium. A reductive elimination from this 18-electrons complex would give the aromatic hydrocarbon and regenerate at the same time the initial catalyst. 

Each of the sandwich compounds forms two isomers, described as clockwise and counterclockwise, respectively. Clockwise means that the atomic sequence in both rings is the same, counterclockwise that the atomic sequence is opposite. The syntheses occur best in THE at -78°C. After warming, the solvent is removed. Purification can be carried out by crystallization from petroleum, ether or better by sublimation at 60-70°C and 10 torr. The yields vary between 25 and 85%. The 17-and 18-electron complexes with V and Fe atoms show the metal atoms to be fixed above and below the ring centers. In contrast, the 19- and 20-electron complexes of Co and Ni possess slipped rings. 

All bonding or nonbonding orbitals are filled resulting in a stable diamagnetic 18-electron complex. Single-electron oxidation to a ferrocenium cation provides a 17-electron species, in which one electron is unpaired. 

The formation of the first osmium hydrido(alkoxo) complex, a yellow air-stable and thermally stable hydrido(methoxo)osmium(II) complex, trans-[OsH(OMe)(Cl) (NO) (P Pr3)2] (72), by the oxidative addition of MeOH to a 16-electron complex, trans-[OsCl(NO)(P Pr3)2] (71) was briefly reported (Eq. 6.22) [51]. 

The complex cp Ir[OC(Ph) =NNCOPh], (195), was obtained from the reactions of cp IrNtBu or [cp Ir(/i-OH)3Ircp ]OH with (PhCONH)2. 0 X-ray structural analysis suggests that the product species should be considered as an electronically unsaturated Ir111, 16-electron complex with normal oxide and amido bonds. The synthesis of the chiral half-sandwich complexes (196) and (197) has been detailed.361... 

The high-resolution EPR spectra of the 17-electron complex (446) have been studied in combination with pulse ENDOR and ESEEM techniques.750... 

Formally, the metal oxidation number x increases to x+2, while the coordination number n of ML, increases to n+2. If such oxidative addition reactions are intended to be the first step in a sequence of transformations, which eventually will lead to a functionalization reaction of C-X, then the oxidative addition product 2 should still be capable of coordinating further substrate molecules in order to initiate their insertion, subsequent reductive elimination, or the like [1], This is why 14 electron intermediates MLu (1) are of particular interest. In this case species 2 are 16 electron complexes themselves, and as such may still be reactive enough to bind another reaction partner. 

We have already mentioned a very strong dyadic association in the formally d5 cobalt complexes such as [Cp Co(dddt)]+ which dimerizes in the solid state to a fully diamagnetic dicationic dyad (Fig. 6a). It represents the extreme situation where the two radicals form a true 2e bond, with the sulfur atom of one dithiolene ligand entering the coordination sphere of the other metal. It should be considered as the consequence of the electron deficiency of these cationic [CpCo(dt)]+ 15-electron complexes. 

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