Molecular ligand exchange reactions

Quantum chemical methods are well established, accepted and of high potential for investigation of inorganic reaction mechanisms, especially if they can be applied as a fruitful interplay between theory and experiment. In the case of solvent exchange reactions their major deficiency is the limited possibility of including solvent effects. We demonstrated that with recent DFT-and ab initio methods, reaction mechanisms can be successfully explored. To obtain an idea about solvent effects, implicit solvent models can be used in the calculations, when their limitations are kept in mind. In future, more powerful computers will be available and will allow more sophisticated calculations to be performed. This will enable scientists to treat solvent molecules explicitly by ab initio molecular dynamics (e.g., Car-Parrinello simulations). The application of such methods will in turn complement the quantum chemical toolbox for the exploration of solvent and ligand exchange reactions. 

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

Attempts have been made to treat selective ion solvation in mixed solvents as ligand exchange reactions [36 e]. We express ion X (cation or anion) existing in solvent A by XA and in solvent B by XB,-. Here, nf=ri if the molecular size of A is very different from that of B or if A is unidentate and B is bidentate (e.g. PC and DME in the solvation of Li+). Otherwise, it is usual that n=n. Then, the ligand exchange reaction in the mixture of A and B will proceed as follows with the increase in the concentration of B ... 

Much more research has been carried out with polymers in which the coordinated metal atom is part of the chain backbone. Typically, the metal atoms are copper, nickel, and cobalt. Oxygen atoms or carbon atoms adjacent to the metal atom provide the electrons required for the coordinate bond.30 Polymers of this type are often rather intractable, for a variety of reasons. Specifically, insolubility can be a problem for species with moderate molecular weights. Also, coordination between chains can cause aggregation, and ligand-exchange reactions with small molecules such as solvents can cause chain scission. However, in some favorable cases, the intramolecular coordination is sufficiently strong for the polymer to be processed by the usual techniques such as spinning into fibers or extrusion into films.30... 

Despite the kinetic lability of the Ln-X-cr-bonds (even the thermodynamically very stable Ln-OR bond is subject to rapid ligand exchange reactions [49]) organolanthanide compounds are thermally very robust over a wide range of temperature (Fig. 5) [114, 116, 139, 144-151]. Thermal stability is not only favorable in catalytic transformations at elevated temperatures [47], for the support of volatile molecular precursors is of fundamental importance in chemical vapor deposition techniques the sublimation behavior is a criterion of thermal stability and suitability for these processes (Fig. 5). 

Some homoleptic unsymmetrical (dmit/mnt, dmit/tdas) dithiolene nickel complex-based D-A compounds with D = TTF and EDT-TTF also exhibit metal-like conductivity (see Table I) (101). Their molecular structure is shown in Scheme 3. The unsymmetrical tetraalkylammonium salts [MLjLJ- (M = Ni, Pd, Pt) have been prepared by ligand exchange reaction between tetraalkylammonium salts of MLj and ML21 (128, 129) and the D-A compounds have been synthesized by electrooxidation. Among these complexes, only the Ni derivatives exhibit metallic-like properties, namely, TTF[Ni(dmit)(mnt)] (metallic down to --30 K), a-EDT-TTF[Ni(dmit)(mnt)] (metallic down to 30 K), TTF[Ni(dmit)(tdas)] (metallic down to 4.2 K), and EDT-TTF[Ni(dmit)(tdas)] (metallic down to --50 K) (see Table I). The complex ot-EDT-TTF-[Ni(dmit)(mnt)J is isostructural (130) to a-EDT-TTF[Ni(dmit)2)] [ambient pressure superconductor, Section II.B.2 (124)]. Under pressure, conductivity measurements up to 18 kbar show a monotonous decrease of the resistivity but do not reveal any superconducting transition (101). 

Coutts and Wailes (52, 53) synthesized a series of 5-coordinate, mono-cyclopentadiene dithiocarbamate complexes of titanium(III) and tita-nium(IV), [CpTi(S2CNR2)2] (R =Me, Et, or Pr), prepared by ligand-exchange reactions between CpTiCl2 and Na(S2CNR2). Infrared spectra and molecular-weight data suggested (52) structure (VI). CpTiXj... 

Numerous applications of [Cr(NH3)6]3+ and its derivatives to mechanistic studies of conventional or photo-assisted ligand-exchange reactions have been extensively reviewed.7,270-276 Among other common Cr111 complexes, [Cr(NH3)6]3+ has been used for the studies of reactivities of muonium and positronium atoms in aqueous solutions.313-315 Several computational methods, including DFT calculations,316 a combination of molecular mechanics and angular overlap model calculations, or vibrational analysis have been used for the prediction and interpretation of electronic spectra and photochemical properties of [Cr(NH3)6]3+. 

GL Rice, SL Scott. Characterization of silica-supported vanadium(V) complexes derived from molecular precursors and their ligand exchange reactions. Langmuir 13 1545-1551,1997. 

Other types of data that have been used, though less extensively, to verify the multiple nature of M—CO bonds are the intensities of carbonyl stretching modes,3 the frequencies of metal—carbon stretching modes,4 equilibrium constants for ligand exchange reactions,5 molecular-orbital calculations6 and electronic spectra.7... 

The random modulation of the electron-nucleus magnetic coupling via electronic relaxation (characteristic time Tg)/ molecular tumbling (characteristic time Tf) and chemical exchange (ligand exchange reaction, characteristic time is assumed to be zero for saturated helicates in eqs (22 - 27)) can be combined in a global correlation time... 

Most of the olefin complexes examined in this study exhibit an unspectacular reactivity towards molecular oxygen, i. e. either ligand exchange reactions, 0-0-bond activation by highly oxophilic metals Sc, Ti, and V, or even complete absence of any reaction are observed (eg. even Cu(C2H4) is unreactive). However, in the case of the iron complexes extensive oxidation reactions are observed. Indeed, not only olefins attached to an iron cation react effectively with molecular oxygen, even stable molecules like benzene and acetone are rapidly oxidized in the presence of Fe+. 

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