Trons- complex

In the case of the tron -complex, only the two chloride ions are substituted, the trans-effect of ammonia being too low to give substitution with the result that white needle crystals of trans-[Pt(NH3)2(tu)2]Cl2 are formed [73]. 

The reverse process, decarbonylation, is also fast but can be arrested by maintaining a pressure of carbon monoxide above the reaction mixture. The reverse of hydrometallation involves the elimination of a hydride from the adjacent carbon of a metal alkyl to form an alkene complex. This process is known as P-hydride elimination or simply P elimination. It requires a vacant site on the metal as the number of ligands increases in the process and so is favoured by a shortage of ligands as in 16-elec-tron complexes. The metal and the hydride must be syn to each other on the carbon chain for the elimination to be possible. The product is an alkene complex that can lose the neutral alkene simply by ligand exchange. So P elimination is an important final step in a number of transition-metal-catalysed processes but can be a nuisance because, say, Pd-Et complexes cannot be used as P elimination is too fast. 

SECM feedback measurements have been applied, together with cyclic voltammetry and controlled potential electrolysis (30), to the reduction of the rhenium aryldiazenido complex [Cp Re(CO)2(/7-N2CsH4OMe)] [BF4] (III(BF4) Cp = T -C5Me5). A one-electron reduction results in the 19-elec-tron complex, IV [Eq. (40)], which then decomposes to give products [Eq. (41)]. 

In the reverse of dissociation, 16-electron species can add a ligand to give 18-elec-tron complexes [19] ... 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

As an example, let s consider an octahedral d] complex, such as one containing a Ti3+ ion. In a free Ti3+ ion, all five 3d-orbitals have the same energy and the d-clec-tron is equally likely to occupy any one of them. However, when a Ti3+ is dissolved in water, six H20 molecules surround it and form a [Ti(H20)h 31 complex. The six point charges representing the ligands lie on opposite sides of the central metal ion along the x-, y-, and z-axes. From Fig. 16.25, we can see that three of the orbitals (dxy, d, and d,x) have their lobes directed between the point charges. These three d-orbitals are called f2g-orbitals. The other two d-orbitals (dz2 and dx, y2), have lobes... 

The reactivities of hydrido(phenoxo) complexes of trons-[MH(OPh)L2] (6 M = Ni 7 M = Pt) (L = phosphine) were examined (Eqs. 6.29, 6.30 Scheme 6-16), and a high nucleophiUdty for the metal-bound phenoxide was suggested [9, 10]. Reaction with methyl iodide produced anisole and trans-[MH(I)L2] for both Ni and Pt complexes. Phenyl isocyanate also provided the insertion products into the metal-phenoxo... 

The IMS response for a compound is strongly dependent on temperature, pressure, analyte concen-tration/vapour pressure, and proton affinity (or elec-tron/reagent affinity). Pressure mainly affects the drift time, and spectral profiles are governed by concentration and ionisation properties of the analyte. Complex interactions among analytes in a mixture can yield an ambiguous number of peaks (less, equal to, or greater than the number of analytes) with unpredictable relative intensities. IMS is vulnerable to either matrix or sample complexity. 

In the Introduction the problem of construction of a theoretical model of the metal surface was briefly discussed. If a model that would permit the theoretical description of the chemisorption complex is to be constructed, one must decide which type of the theoretical description of the metal should be used. Two basic approaches exist in the theory of transition metals (48). The first one is based on the assumption that the d-elec-trons are localized either on atoms or in bonds (which is particularly attractive for the discussion of the surface problems). The other is the itinerant approach, based on the collective model of metals (which was particularly successful in explaining the bulk properties of metals). The choice between these two is not easy. Even in contemporary solid state literature the possibility of d-electron localization is still being discussed (49-51). Examples can be found in the literature that discuss the following problems high cohesion energy of transition metals (52), their crystallographic structure (53), magnetic moments of the constituent atoms in alloys (54), optical and photoemission properties (48, 49), and plasma oscillation losses (55). 

A trons-[RuCl2(diphosphine)(l,2-diamine)] complex with (R,R)-Et-DuPhos and (R,R)-l,2-diaminocyclohexane as the ligand combination has been found to be effective for the hydrogenation of imine 143, with up to 94% ee being obtained under the standard basic conditions employed for this catalytic system [198]. Unfortunately, the optimum combination of chiral diphosphine and diamine was found to be substrate-dependent, with only 40% ee being obtained for 2-methylquinoxaline 144 with Et-DuPhos. 

Because of the retained isocyano functionality, the dihydropyridone MCR product 85 can be used in various follow-up (multicomponent) reactions. For example, the Passerini reaction between 85, a carboxylic acid, and an aldehyde or ketone produces a series of dihydropyridone-based conformationally constrained depsipeptides 86 [171]. The subsequent Passerini reaction could also be performed in the same pot, resulting in a novel 6CR toward these complex products containing up to seven points of variation. Reaction of 85 with an aldehyde or ketone and amine component resulted in the isolation of dihydrooxazolopyridines (DHOPs, 87) [172] via a similar approach as the 2,4,5-trisubstituted oxazole variant toward 42 reported by Tron and Zhu (Fig. 15) [155]. The corresponding DHOPs (87), which... 

These expressions have been applied to the series isocyanide and carbonyl complexes of rhenium(I) tr ns-[ReL2(dppe)2]" (L = CNR, CO) [19, 20] and of nitrile and carbonyl compounds of iron(II) tmns-[FeL2(depe)2]2+ (L = NCR, CO) [35], and the Es and /J values of the corresponding trons- ReL(dppe)2 " and trans- FeL(depe)2 " " centers, which are given in Table 9, have been discussed earlier. 

Side reactions specific to one component play an important role in the reforming of a mixture. For example, aromatics are more prone to coking upon reforming, so their presence in a mixture can lower syngas yields over time due to catalyst deactivation. Also, the catalyst surface-component interactions may play an important role in the reforming of a mixture. For example, aromatics have an abundance of 71-electrons, so they may occupy active sites for a longer duration, due to 71-complexation between d-orbitals of the metal and 7i-elec-trons. Hence there will not be enough reactive sites available for the desired reaction to occur. 

---