Reactivity of Coordination Complexes

The kinetics and thermodynamics of the addition reactions of amine and N-heterocyclic ligands to [0s04] have been studied using UV/Vis and NMR spectroscopies. With py, 4-pic, and bpy, the kinetics are too fast to be studied without using stopped-flow techniques, whereas the 

Another important factor that separates zr-acceptor ligands in traditional organometallic complexes, as compared to pentaammine com- 

Spontaneous Aquation Rates of Pentaammineosmium(III/II) and Analogous Pentaammineruthenium(III/II) Complexes  

Aqua exchange in [Os(t 6-C6H6)(OH2)3]2+ has been subjected to detailed mechanistic studies, along with the Ru analog. The volumes of activation are small and positive. This has been taken as an indication of an interchange mechanism that is near the middle of the continuum, but that is more dissociatively than associatively activated (620). 

Electron Transfer Reactions 1. Inter molecular Electron Transfer 

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The mechanism of conjugate addition reactions probably involves an initial complex between the cuprate and enone.51 The key intermediate for formation of the new carbon-carbon bond is an adduct formed between the enone and the organocopper reagent. The adduct is formulated as a Cu(III) species, which then undergoes reductive elimination. The lithium ion also plays a key role, presumably by Lewis acid coordination at the carbonyl oxygen.52 Solvent molecules also affect the reactivity of the complex.53 The mechanism can be outlined as occurring in three steps. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

Interest in the synthesis and reactivity of coordinatively unsaturated low-valent metal complexes has led to the use of an o-carboranedithiolato ligand in the formation of metalladithiolene ring complexes. Recently, we69 70 and Wrackmeyer et al.1 72 have reported on the synthesis of the 16e cobalt, rhodium, and iridium... 

Redox Reactivity of Coordinated Ligands in Pentacyano(L)Ferrate Complexes Jose A. Olabe... 

Also, it is worth mentioning (a) the process of N=N triple bond cleavage by Mo(III) complex [6], and (b) the reactivity of coordinated dinitrogen with electrophiles, including the protons and organic free radicals [7], and coordinated [8] and free [9] dihydrogen. 

The vast majority of the circumstantial evidence concerning this reaction points toward the presence of i74-vinylketene complexes such as E-18 at some point along the reaction coordinate. Moreover, the chromacyclohexa-diene pathway cannot fully explain all of the data concerning the benzannu-lation reaction.13 The accumulation of evidence for the intermediacy of rf-vinylketenes has been recorded elsewhere,8,10,11,13,27 but will be considered here briefly to give an impression of the general methods that have been used to elucidate the reactivity of vinylketene complexes. 

Having established a very effective method for the synthesis of tricar-bonyl(T74-vinylketene)iron(0) complexes, Thomas has subsequently undertaken the most comprehensive study on the reactivity of these complexes to date. The reactions of 221 with phosphoramidate anions,90134 coordinating ligands such as phosphines3 and isonitriles,69,87,89,135,142,143 a variety of nucleophiles,86,89135142 phosphonoacetate anions,88,89 alkynes,108,109,144,145 and al-kenes146,147 have ah been investigated. Crucially, Thomas has also developed a method138 for the kinetic resolution of the vinylketene complexes (221) that ultimately yields enantiomerically pure samples of the complex. This... 

In this chapter, we first analyzed the electronic structures of metal vinylidene and allenylidene complexes. The electronic structures allow us to understand the reactivities of these complexes. For metal vinylidene complexes of the Fischer-type, nucleophilic attack usually occurs at the a-carbon and electrophilic attack at the P-carbon. For the corresponding metal allenylidenes, electrophilic attack occurs at the P-carbon and/or the metal center. Then we briefly reviewed the theoretical study of the barriers ofrotation ofvinylidene ligands in various flve-coordinate complexes M (X) C1(=C=CHR)L2 (M = Os, Ru L = phosphine). The study showed that 7t-acceptor ligands (X), electron-withdrawing substituents and lighter metals gave smaller barriers. 

While most superoxo complexes—in contrast to peroxo compounds— have been assigned a bent, end-on coordination mode [9], the superoxide ligand of Tp Cr(02)Ph was suggested to exhibit the more unusual side-on (r] ) coordination [10]. The reactivity of the complex did not allow for the determination of its molecular structure however, close analogs could be isolated, crystallized and structurally characterized by X-ray diffraction. For example, the reaction of [Tp Cr(pz H)]BARF (pz H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis(3,5-bis(trifiuoromethyl)phenyl)borate) with O2 produced the stable dioxygen complex [Tp Cr(pz H)( ] -02)]BARF (Scheme 3, bottom), which featured a side-on bound superoxide ligand (do-o = 1.327(5) A, vo-o = 1072 cm ) [11]. Other structurally characterized... 

REDOX REACTIVITY OF COORDINATED LIGANDS IN PENTACYANO(L)FERRATE COMPLEXES... 

The present volume is a non-thematic issue and includes seven contributions. The first chapter byAndreja Bakac presents a detailed account of the activation of dioxygen by transition metal complexes and the important role of atom transfer and free radical chemistry in aqueous solution. The second contribution comes from Jose Olabe, an expert in the field of pentacyanoferrate complexes, in which he describes the redox reactivity of coordinated ligands in such complexes. The third chapter deals with the activation of carbon dioxide and carbonato complexes as models for carbonic anhydrase, and comes from Anadi Dash and collaborators. This is followed by a contribution from Sasha Ryabov on the transition metal chemistry of glucose oxidase, horseradish peroxidase and related enzymes. In chapter five Alexandra Masarwa and Dan Meyerstein present a detailed report on the properties of transition metal complexes containing metal-carbon bonds in aqueous solution. Ivana Ivanovic and Katarina Andjelkovic describe the importance of hepta-coordination in complexes of 3d transition metals in the subsequent contribution. The final chapter by Sally Brooker and co-workers is devoted to the application of lanthanide complexes as luminescent biolabels, an exciting new area of development. 

The problem of to what extent and how the reactivity of the isonitrile ligand is altered by coordination to a metal has an important bearing on the function of transition metals as homogeneous catalysts. It is also of interest from the standpoint of energy transfer in vivo, to determine whether or not there is any correlation between the reactivities of isonitrile complexes and their conductivities. In this paper we summarize some recent work on the syntheses and reactions of iron isonitrile complexes. 

Attempts have been made to investigate the electrophilic substitution reactivity of coordinated aniline relative to the free ligand. For CrCl3 (an)3, little rate enhancement was observed for bromina-tion reactions, but extensive complex decomposition accompanied the substitution.895 Complexes such as m-CoCl(en)2(an)2+ have abnormally high base hydrolysis rates when compared with their alkylamine analogues.15... 

A tetracobalt anionic complex, viz. [In Co(CO)4 4] (27) (37,37a), has been briefly described together with the thallium analogue (28) (37a), both formed by addition of [Co(CO)4] to either 25 or 26. No structural details have been reported although the indium and thallium centers are presumably tetrahe-drally coordinated by the four cobalt atoms. Mention is also made (37a) of the facile heterolytic bond dissociation (In—Co or Tl—Co) observed in polar solvents. Little has been reported about the reactivity of these complexes, although a discussion on the use of 25 as a catalyst in the dimerization of norbornadiene has appeared (58). 

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