Transition metal complexes coordination isomerism

There are few reports of the transition metal complex-catalyzed isomerization of S-allyl sulfides and sulfones. This is clearly a consequence of the very strong coordinating ability of sulfur atoms and the resulting tendency for S-C (allyl) bond cleavage. In the case of a bulky substituent being present at the sulfur atom, the isomerization to 1-propenyl derivatives is successful (Eqs. 12.14 and 12.15) [20]. 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

There is considerable and widespread interest in the metal complexes of these anions and current research topics comprise for example (i) the spectroscopic study of the binding in these anions (linkage isomerism) and their complexes, (ii) the synthesis of regular polymers of their transition metal complexes and study of the semiconducting properties of these polymers, (iii) the use of the pseudohalides in pharmacological (e.g. low toxicity of —SCN) and biochemical studies (easy complexation of SCN- to metals), and (iv) the use of the activation of these triatomic anions by coordination to metals for their selective conversion in organic synthesis. 

The transition metal complex-catalyzed formation of 1,3-dioxepanes from vinyl ethers has also been described. For example, reaction of allyl vinyl ether 157 with a nonhydridic ruthenium complex at higher temperatures and without any solvent produced 1,3-dioxepane 159 whereas, the use of a hydridic ruthenium complex resulted in the formation of vinyl ether 158 by double-bond isomerization (Scheme 43). It was suggested that cyclic acetal formation proceeds via a 7i-allyl-hydrido transient complex, which undergoes nucleophilic attack of the OH group at the coordinated Jt-allyl <2004SL1203>. 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

The discovery that the nitrosyl ligand is capable of binding to transition metals in two isomeric valence forms1 is one of the most dramatic recent developments in organometallic chemistry. Since bent NO donates 2 fewer electrons to the metal than the linear isomer does, linear-bent tautomerism raises the possibility of coordinative unsaturation and catalysis.2 In fact, complexes of nitric oxide are receiving increasing attention as catalysts,3 since they are more reactive than the corresponding carbonyls.4... 

Isomerization of olefins—redistribution of carbon-carbon double bonds—often occurs in the presence of transition metal complexes such as metal earbonyls, platinum group metal ions, and silver(I) compounds. It appears that sueh catalytic activity has two requirements. (1) The metal complex must contain labile ligands that can easily be displaced by an olefin to form a vr-complex or must be able to easily expand its coordination. (2) The metal complex must be kinetically labile so tha t the isomerized olefin can be displaced by another olefin to continue the cycle. 

The ability of the nitrosyl ligand to behave as an electron pair reservoir has also been considered to play an important part in certain catalytically active systems. The vacant site provided by isomerization of the ligand could enable an unsaturated organic molecule to enter the transition metal s coordination sphere, thus forming an active intermediate. Examples of catalysis by nitrosyl complexes include the hydrogenation of alkenes by Rh(NO)L3 species and the dimerization of dienes in the presence of Fe(CO)2(NO)2 or Fe(n-C3Hs)(CO)2NO. Certain molybdenum dinitrosyl complexes, such as MoCljfNOljfPPhjlj, have also been found to provide very efficient alkene dismutation catalysts. ... 

The second series of related materials was based on the (3,4-didodecyloxyphenyl)diaminodi-oxime derivative (73). Vicinal dioximes are capable of coordinating through N,N or N,0 sites of the oxime groups. Upon the complexation, the ligands fold back, and the transition metal complexes are V,V-coordinated with a square-planar structure. Both isomeric forms were selectively isolated for the nickel complexes, but only the anii-isomer for the palladium complex. The two uwti-complexes complexes exhibited a Colh phase ((73) M = Ni Cr 78 Colh H7 I M = Pd Cr 80 Colh 131 I). Most astonishing, however, was the observation of a Colh phase for the isomeric amphi-mckel complex ( , Z-isomer), between 66 °C and 145 °C. Not only was the existence of mesomorphism surprising and unexpected, but the mesophase stability also increased with respect to the anti-complex. 

The intramolecular insertion of a hydride into a coordinated olefin is a crucial step in olefin hydrogenation catalyzed by late transition metal complexes, such as those of iridium, rhodium, and ruthenium (Chapter 15), in hydroformylation reactions catalyzed by cobalt, rhodium, and platinum complexes (Chapter 16), and in many other reactions, including the initiation of some olefin polymerizations. The microscopic reverse, 3-hydride elimination, is the most common pathway for the decomposition of metal-alkyl complexes and is a mechanism for olefin isomerizations. 

Although many transition metal complexes catalyze olefin isomerization, early studies have shown that the presence of silane markedly affects such reactions that occur concurrently with hydrosilylation. Double-bond migration, which is a characteristic feature of most coordination catalytic reactions, can be illustrated schematically in (Scheme 6) as a side reaction occurring during hydrosilylation (3,10,47). 

The most general route to generate vinylidene complexes [M]=C=C(H)R is the direct activation of terminal alkynes HC=CR by a coordinatively unsaturated transition metal complex, via the generation of unstable 77 -alkyne or hydride-alkynyl intermediates which tautomerize into the thermodynamically more stable vinylidene isomers (Scheme 15) ia,ic,207 theoretical and kinetic studies on the metal-mediated alkyne-vinylidene isomerization... 

The reactivity of phosphasilenes with organic compounds, for example, [2 + 2]-cycloaddition reaction (8) has been intensively investigated. The reactivity of phosphasilenes with transition metal complexes, however, is comparatively unexplored (9,10). The described phosphasilene 1 can imdergo unprecedented EtZ isomerization of the Si=P bond upon coordination to tungsten (11). Another remarkable reactivity study describes the coordination of group 10 transition metals to phosphasilene 1 generating dinuclear platinum and palladium complexes with Si—P bond cleavage and a bissilylene nickel complex from a Ni(0) precursor (12). 

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