Metal Bonded Systems

Oligomers and polymers containing backbone Pt-Pt bonds have been reported via reactions of a platinum complex with diacetylides and diisocyanides (scheme 37). Oligomers wiffi low solubility were formed in die reactions with diacetylides. IR studies on ffie polymers formed with reaction of diisocyanides did not exhibit imco-ordinated isocyanide groups. 

Polyurethanes, amides, and ureas containing photodegradable metal-metal bonds in the backbones were reported by Tenhaedd and Tyler in the early 

Royo and coworkers reported the synthesis of polymers containing bridging molybdenum bonds (scheme 39). Complexes containing anj-Mo(CO)3Cl moieties were used in the reaction with MeMgCl to give the Mo-Mo bonds. Polymer 130 exhibited poor solubility in organic solvents. 

Polysiloxanes containing Fe-Fe bonds have been reported using two methodologies. The first methodology reacted the polysiloxanes with Fe(CO)5 and the second reacted a diiron Cp2Fe2(CO 4 complex, containing SiMc2NMe2 funcitons with disilanols. The second method is shown in scheme 40. 

Reactions of dicobaltoctacarbonyl with a silicon-based polymer with aUcyne emits in the backbone have led to the isolation of polymers containing Co-Co bonds (scheme 41) °  

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Alkylaminobis(difluorophosphines) novel bidentate ligands for stabilizing low metal oxidation states and metal-metal bonded systems. R. B. King, Acc. Chem. Res., 1980,13, 243-248 (41). 

The structural analogies between metal oxides, metal-alkoxides and metal-alkoxide-oxide polymers originally noted for d° metal systems can be extended to metal-metal bonded systems. 

The currently available quantum chemical computational methods and computer programs have not been utilized to their potential in elucidating the electronic origin of zeolite properties. As more and more physico-chemical methods are used successfully for the description and characterization of zeolites, (e.g. (42-45)), more questions will also arise where computational quantum chemistry may have a useful contribution towards the answer, e.g. in connection with combined approaches where zeolites and metal-metal bonded systems (e.g. (46,47)) are used in combination. The spectacular recent and projected future improvements in computer technology are bound to enlarge the scope of quantum chemical studies on zeolites. Detailed studies on optimum intercavity locations for a variety of molecules, and calculations on conformation analysis and reaction mechanism in zeolite cavities are among the promises what an extrapolation of current developments in computational quantum chemistry and computer technology holds out for zeolite chemistry. 

Low frequency Raman spectra of metal-metal bonded system (includes Fe2(CO)9 with k(CO), Fe3(CO)12, (n5-CsHsFe(CO)2]2 and (tjs-C5H5Fe(CO) 4... 

Finally — and this has to do with the nature of the bond strengths in such metal-metal bonded systems — the dimer also thermally dissociates. With a very reactive halogen atom donor,... 

The overall performance of a polymer-metal bond system is affected by ... 

The weakness of most metal-metal bonds compared with metal-ligand bonds makes cleavage of metal-metal bonds by nucleophiles a common process (15). In the case of metal-metal double bonds this corresponds to nucleophilic addition to the metal-metal bonded systems. Since unsaturated clusters exist which can be considered to contain metal - metal double bonds, this should be an important aspect of substrate activation by clusters. 

There are no routes yet to homoleptic metal isocyanide anions. If one considers the interesting products obtained from methyl iodide additions to molybdenum (43) and manganese (44) carbonyl isonitrile anions, negatively charged isocyanide complexes should have some interesting chemistry. Also, now that a simple route to [CpFe(CNR)2]2 complexes has been devised (45), the synthesis of the anion [CpFe(CNR)2] could provide a route to a range of products including heterometal-metal bonded systems. 

This review summarizes some earlier qualitative work as well as recent quantitative studies of redistribution equilibria and describes the principles underlying the mathematical treatment of such equilibria as well as the general implications of these equilibria with respect to general chemistry. In line with the general objective of the Advances in Organometallic Chemistry series, this article is limited to carbon-metal-bonded systems, metal hydrides, metal carbonyl compounds, metallocenes, and similar complexes. Excluded therefore are halogen-, sulfur-, nitrogen, and phosphorus-based systems.Various aspects of redistribution reactions were reviewed previously (42, 74, 87, 88,150,186, 285, 286, 288). 

The reaction favoring ionic rather than neutral products is controlled by a fine balance of factors, such as temperature, solvent, nature of ligand, and character of the metal-metal bonded system. As yet, the relative importance of these factors remains undefined. 

If the type of bonding and the electronic structure of metal-metal-bonded systems rather than exclusively structural aspects are the aim of a study, then molecular mechanics alone is not appropriate. Recent studies with broken-symmetry DFT approaches have yielded structural results that are in excellent agreement... 

The lack of Rh" complexes generated from the reduction of Rh111 species was noted by Taube in 1968362 and in his recent review Felthouse notes that electrochemical studies of other metal-metal bonded systems have been a fertile area for study... although relatively few studies have been reported on dirhodium(II) compounds. 282 That is rapidly changing, as Rh2 electrochemistry has become a vigorously pursued research area. 

Reduchve elimination of hydrocarbons or their derivatives is another viable route to metal-metal bonded systems. A good example is dinuclear reductive elimination, as studied by Bergman and coworkers (equation 53). In this case, both the single and triple-bonded complexes are formed. Exposure to CO instantly converts the triple-bonded complex to the single-bonded one. 

Similar reactions can occur between radicals and hydrides and involve H-atom transfer, followed ultimately by formation of the most stable metal-metal bonded systems (equation 59). The mixed-metal complex formed slowly disproportionates (equation 60). 

The photochemistry of metal-metal bonded complexes has been the subject of several reviews see Luminescence Behavior Photochemistry of Organotransition Metal Compounds and Photochemistry of Transition Metal Complexes Theory). Strong a bonding to a antibonding transitions are often observed in metal - metal bonded systems. These lead to photogenerated fragments that undergo reactions and may ultimately recombine (equation 72). 

These reactions are often reversible and depend on temperature, concentration, and nature of the base. In addition, since this reaction involves changing from a metal-metal bonded system that is soluble in organic solvents to an ionic complex that is water soluble, the solubility patterns change greatly. Phase transfer catalysis see Phase Transfer... 

There are three features that are different for metal-metal bonded systems than for mononuclear complexes possible substitution at different metals, fragmentation, and the effect of the metal metal bond on reactivity. 

J. L. Eglin, Dinuclear Metal-metal Bonded Systems,... 

Future progress may, however, be accelerated by recent and forthcoming developments. Recent flash photolysis studies of metal-carbonyls and metal-metal bonded systems should provide a foundation for more extensive development of this area. Applications of picosecond and nanosecond flash systems to a wide variety of problems is expected. Such techniques should be... 

Chapter 9 covers binuclear metal-metal bonded systems and acts as a link between the mononuclear species and the cluster chemistries of Chapters 10-12. The rapid evolution of the latter aspects is another highlight of development of the chemistry of Groups 4-6. 

Dinuclear group 3-6 metal-metal bonded systems encompass a wide range of compounds with a variety of ligand types. With the use of innovative starting materials or novel synthetic methods, the number of new complexes continues to expand. Several general trends are apparent for known... 

The present review of metal-metal bonded systems focuses primarily on complexes that are supported by structural evidence. Information for group 6 compounds concentrates on complexes synthesized since 1990. Although several organometallic complexes are discussed, their inclusion is primarily a result of either their use as a starting material or an unusual oxidation state or structure. 

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