Coordinated ligands Rearrangement

Tetrahaptocyclohexatriene- and cyclooctatetraene-metal-tricarbonyl complexes undergo some unusual cyclo-addition reactions with tetracy-anoethylene in which the dienophile undergoes a 1,3 addition across the coordinated ligand and causes a rearrangement of the metal-carbon bonds, for example (99),... 

A different type of rearrangement involves cycloaddition of dimethyl acetylenedicarboxylate on to a copper complex which serves to activate the ligand (Scheme 73).229 The reaction is similar in some respects to that shown in equation (42), and indicates the difficulty in the categorization of many of the reactions of coordinated ligands according to mechanism. Consequently, the above account is of necessity somewhat subjective. 

In general then, activation of C02 by coordination to the metal is apparently not required for insertion of C02 into a metal-amide or metal-alkox-ide bond. Instead, reaction between free ligand and C02 in solution may take place, or the C02 may attack directly the coordinated ligand, followed by a rearrangement to yield the product. 

The addition of phenylenediammonium to a mixture of 19 and 21 thus caused two distinct rearrangements to occur initial (covalent) imine exchange followed immediately by (coordinative) ligand exchange, resulting in the exclusive formation of mixed-ligand complex 22. 

For Cu 7Cu systems, the electron transfer takes place with a large rearrangement of the inner coordination sphere, as Cu generally prefers distorted octahedral or square pyramidal environments whereas Cu likes better a tetrahedral geometry. This consideration implies that, in unconstrained systems, one or two Cu-S bonds must be broken and bond angles must be drastically modified upon Cu reduction. Therefore, any steric constraint imposed by the coordinated ligand could strongly affect the kinetics of the Cu 7Cu electron transfer process. 

Geometrical Photoisomerizations. in this section geometrical photoisomerization reactions of OTM compounds are considered. Recent accounts of photoinduced rearrangements of coordinated ligands and OTM complex photoassisted isomerization of olefins, dienes, and so on are available elsewhere (3,4,91). 

Reaction of the Mo Schrock carbyne complex in equation 10.58 with HBF4 results in protonation at Ccaibyne, followed by rearrangement to the thermodynamically more stable Mo-H complex. The BF4 ion is such a weakly coordinating ligand that substitution at the metal does not occur.98... 

Dinucleating carboxylates support diiron(II) complexes that bind O2 rapidly in a simple bimolecular process (first order in the complex and first order in O2).92 The activation parameters estimated for the oxygenation of the XDK-Im complex, approximately A/T = 16 kJ/mol and AS = —120 J/(molK) are in accord with the values obtained for other diiron(II) complexes and correspond to a low-barrier O2 coordination at a vacant iron(II) center. A mechanism proposed for the oxygenation reaction includes the attack of the O2 molecule on the unsaturated (five-coordinate) iron(II) center concomitant with the carboxylate shift at this center followed by the coordination to the second, six-coordinate iron(II) center and additional ligand rearrangement (Figure 4.26). 

The papers reviewed here can be divided into two broad categories (i) those that are fairly non-specific in their choice of data, studying basically all complexes with a common stoichiometry ML , where M = metal, L = a coordinated ligand atom, and rt = 3, 4, 5,..., 12, and (ii) those that set out to examine specific reaction paths - such as metal cluster rearrangements, geared rotations, ring whizzing, etc. - and which are fairly selective in data retrieval. We shall discuss the papers in the above order. 

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