Complexes of NO and

Mulliken s general concept of charge-transfer complexes can be given explicit and quantitative reformulation in the NBO framework. This allows one to recognize the essential electronic continuity that relates CT complexes of different types, including H-bonded species (n-a CT complexes). Particular attention was paid to the interesting 7t-7t CT complexes of NO+ and related pi-acids, which exemplify the distinctive quantal dependence on the shapes of donor and acceptor orbitals. 

These studies at the same time aroused my interest in the mechanistic aspects of the reaetions, including the complexes of RCOF and RF with BF3 (and eventually with other Lewis acid fluorides) as well as the complexes they formed with aromatics. 1 isolated for the first time at low temperatures arenium tetrafluoroborates (the elusive (T-complexes of aromatic substitutions), although I had no means to pursue their structural study. Thus my long fascination with the chemistry of car-bocationic complexes began. 

The coordination chemistry of NO is often compared to that of CO but, whereas carbonyls are frequently prepared by reactions involving CO at high pressures and temperatures, this route is less viable for nitrosyls because of the thermodynamic instability of NO and its propensity to disproportionate or decompose under such conditions (p. 446). Nitrosyl complexes can sometimes be made by transformations involving pre-existing NO complexes, e.g. by ligand replacement, oxidative addition, reductive elimination or condensation reactions (reductive, thermal or photolytic). Typical examples are ... 

In contrast to the numerous complexes of NO which have been prepared and characterized, complexes of the thionitrosyl ligand (NS) are virtually unknown, as is the free ligand itself. The first such complex [Mo(NS)(S2CNMc2)3] was obtained as orange-red air-stable crystals by treating [MoN(S2CNMe2>3] with sulfur in... 

If B— [bij] is an N xN matrix in which bu equals the degree of vertex i, bij = —1 if vertices i and j are adjacent and bij = 0 otherwise, then the number of spanning tree of G is equal to the determinant of any principal minor of B [hararybO]. The extremes occur for totally disconnected graphs that have no spanning trees and thus a complexity of zero, and for complete graphs of order N that contain the maximum possible number of distinct trees on N vertices. ... 

The results were simple and clear-cut Only the two terms ofa° and Emin were involved for the a-cyclodextrin systems, and the two terms of k and Emin, for (S-cyclodextrin systems. This means that the stabilities of the inclusion complexes are mainly governed by the electronic and steric interactions in a-cyclodextrin systems and by the hydro-phobic and steric interactions in (i-cyclodextrin systems, regardless of the position of the substituents in the phenols. These observations agree well with those by Harata23), who showed that there is no appreciable difference in thermodynamic parameters between cyclodextrin complexes of m- and p-di substituted benzenes and that the contribution of the enthalpy term to the complexation is more significant in a-cyclodextrin systems than in P-cyclodextrin systems, where the inhibitory effect... 

It was proposed [445 - 447] that the dissolution of tantalum and niobium oxides in mixtures of hydrofluoric and sulfuric acids takes place through the formation of fluoride-sulfate complexes, at least during the initial steps of the interaction and at relatively low acid concentrations. Nevertheless, it was also assumed that both tantalum and niobium fluoride-sulfate complexes are prone to hydrolysis yielding pure fluoride complexes and sulfuric acid. No data was provided, however, to confirm the formation of fluoride sulfate complexes of tantalum and niobium in the solutions. 

Pyridine complexes of Pd- and Pt-pincer ligands are also suitable substrates for olefin metathesis [116]. The first-generation catalyst 9 efficiently mediates the RCM of diallylphosphines and diallyl sulfide when the heteroatom is com-plexed by a cationic [C5H5(NO)(PPh3)Re] moiety [117]. This principle has been exploited in the same study for tungsten, rhodium, and platinum complexes. 

The cyclopentadienyl triflate complexes of zirconium and titanium 51 and 52 (Figure 3.7) are also active catalysts [51]. Their activity has been tested in a wide variety of dienes and dienophiles. It is noteworthy that even at low catalyst loadings, rate accelerations between 10 and > 10 times have been observed. No special precautions were taken to dry the solvents or the substrates, in contrast with the traditional Lewis acids which require either predried solvents or high catalyst loadings. 

Attempts to exploit the reaction of the dianion with alkyl halides to produce a c/.v-dialkyl complex by using 1,2- or 1,3-dihaloalkanes did not indeed give this result. The reaction of Ru(Por) " with 1,2-dibromoethane was sucessful, but the resulting metallacyclopropane product is better formulated as a /r-complex of ethene, and will be discussed below in the section on alkenc and alkyne complexes. The corresponding reaction of the diiinion with 1,3-dichloropropane gave no evidence for a metallacyclobutane. but instead free cyclopropane was detected by GC analysis and the porphyrin product was Ru(TTP)(THF)2. ... 

More important than this, however, is the fact that until recently there have been no substantial differences observed in the chemistry of metal complexes of alkyl and aryl isocyanides. In general, the choice of which isocyanide to use seems to be made largely at random, dictated perhaps by convenience as much as any other factor. Lacking any substantial chemical differences between the two groups of complexes, one might wish to minimize, rather than emphasize, this comparison. However, several observations have recently been made which seem to substantiate the earlier conclusion. 

The SCR catalyst is considerably more complex than, for example, the metal catalysts we discussed earlier. Also, it is very difficult to perform surface science studies on these oxide surfaces. The nature of the active sites in the SCR catalyst has been probed by temperature-programmed desorption of NO and NH3 and by in situ infrared studies. This has led to a set of kinetic parameters (Tab. 10.7) that can describe NO conversion and NH3 slip (Fig. 10.16). The model gives a good fit to the experimental data over a wide range, is based on the physical reality of the SCR catalyst and its interactions with the reacting gases and is, therefore, preferable to a simple power rate law in which catalysis happens in a black box . Nevertheless, several questions remain unanswered, such as what are the elementary steps and what do the active site looks like on the atomic scale ... 

Wiberg has reviewed this oxidation and suggests that the complex of Mason and Kowalak is a mixed anhydride, cf. the oxidations of sulphur dioxide (p. 285) and formic acid (p. 316). The anhydride cannot be of neutral charge, however, as no acidity dependence is observed and a possibility is... 

A quantitative consideration on the origin of the EFG should be based on reliable results from molecular orbital or DPT calculations, as pointed out in detail in Chap. 5. For a qualitative discussion, however, it will suffice to use the easy-to-handle one-electron approximation of the crystal field model. In this framework, it is easy to realize that in nickel(II) complexes of Oh and symmetry and in tetragonally distorted octahedral nickel(II) complexes, no valence electron contribution to the EFG should be expected (cf. Fig. 7.7 and Table 4.2). A temperature-dependent valence electron contribution is to be expected in distorted tetrahedral nickel(n) complexes for tetragonal distortion, e.g., Fzz = (4/7)e(r )3 for com-... 

Tinnemans et al.132 have examined the photo(electro)chemical and electrochemical reduction of C02 using some tetraazamacrocyclic Co(II) and Ni(II) complexes as catalysts. CO and H2 were the products. Pearce and Pletcher133 have investigated the mechanism of the reduction of C02 in acetonitrile-water mixtures by using square planar complexes of nickel and cobalt with macrocyclic ligands in solution as catalysts. CO was the reduction product with no significant amounts of either formic or oxalic acids... 

There is no experimental evidence for 7r-complexation of arenes to gold centers in the condensed phase. Quantum-chemical calculations were carried out on various levels of theory for 1 1 and 2 1 complexes of benzene and substituted benzenes with Au+ in the gas phase. For all model systems investigated, it has been predicted that an -coordination to a single carbon atom is the ground state of the cation (Cy-symmetry for [(C6H6)Au]+ and 6 -symmetry for [(C6H6)2Au] ). Similar results were obtained for hexafluorobenzene, for which a coordina-... 

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... 

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