Ligand-dissociation

The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

The unstable pale blue-green bis(2,276, 2 -terpyridine)iron(3+) ion [47779-99-7], [Fe(terpy)2], has been obtained by oxidation of [Fe(terpy)2]. It is very unstable with respect to reduction by solvent and ligand dissociation. The perchlorate salt [2153642-5] has been reported. 

CN > NO2 > NH3 > H2O, F > Cl . Exceptions do occur. Photochemical Ligand dissociation is useful in the synthesis of multinuclear metal complexes such as diiron nonacarbonyl [15321-51 from iron pentacarbonyl [13463-40-6]... 

These are generally analogous to those of Wilkinson s compound, with the important difference that ligand dissociation cannot occur, so that the product of oxidative addition with H2 cannot have a vacant site to bind an alkene and will thus not act as a hydrogenation catalyst [53]. 

An understanding of these LR dynamics requires both biological and mathematical insight. In this chapter we define these biological processes in terms of RG mechanisms and consider whether these processes reflect receptor in different states of activity. Next we develop a mathematical description to account for at least six separate processes (1) ligand association, (2) rapid ligand dissociation, (3) slower dissociation, (4) internalization, and (5 and 6) two different Interconversions among receptor forms. 

Adtveoitages. The antibody technology is remarkably convenient. It requires only that a significant fraction of FLPEP be receptor bound ( 10%). The assay can be set up in a few minutes and is applicable to membranes, permeabilized cells, cells, or any other preparation in which the receptor concentration is in the range of 1 niV or greater. We use this assay routinely to analyze ligand dissociation kinetics. 

The existence of copper(I) isocyanide complexes is well documented, of course (90). Such complexes are basically straightforward, having stoichiometries and physical and chemical properties analogous to other copper(I) complexes. It would be somewhat surprising if the studies currently underway on the catalytic systems had not attempted to sketch in this relationship more precisely. No copper(O) isocyanide complexes are known, so such species if they exist here would be particularly interesting their stability is clearly low with respect to ligand dissociation, or they would have been isolated in these studies. 

For each of these reactions kinetic data were obtained. The reactions were first order in complex concentration, and zero order in isocyanide, as expected. The complex Ni(CNBu )4, and presumably other Ni(CNR)4 complexes as well, undergo ligand dissociation in solution. In benzene solution, a molecular weight determination for this compound gives a low value (110). This is in accord with the presumed mechanism of substitution. 

Ni — N bond breaks and one NH2 group of an ethylenediamine ligand dissociates from the metal complex. 

An unusually slow relaxation has been observed for the 2,6-pyridine-dicarboxaldimine cobalt(II) complex [Co(2,6-(CH3NH=CH)2py)2](PFg)2 in solution. Thus a relaxation time -c = 83 ns has been reported [99], the rate constants being among the lowest found. It has been suggested that nonelectronic factors such as partial ligand dissociation, steric effects or solvent interaction may be rate determining in this equilibrium. 

Upon recrystallization, [Ni(tpzlmtacn)]2+ affords [Ni(L)(MeCN)]2+ (L = l,4-bis(pyrazol-l-ylmethyl)-l,4,7-triazacyclononane) via a N-dealkylation reaction and loss of a pendent arm.1420 More rational routes to Ni complexes of tacn ligands with only one or two pendent arms have been developed.1431,1432 In [Ni(L)(X) ]x (e.g., L= l-(3-aminopropyl)-l,4,7-triazacyclononane (n = 2) or l-(l-methylimidazol-2-ylmethyl)-l,4,7-triazacyclononane (n = 2) or l,4-bis(l-methyl-imidazol-2-ylmethyl)-l,4,7-triazacyclononane (n = 1), the coordination sphere is completed by additional ligands that bind either terminal (X = C1 , H20) or bridge two metal ions (X = N3 , OH, oxalate). The Ni11 complex of l,4-bis(2-pyridylmethyl)-l,4,7-triazacyclononane has been shown to be extremely inert to ligand dissociation in aqueous solution.1433 In (562), the tacn ligand provides a single bidentate arm.1434... 

Sources of catalytically active palladium(O) typically arise from ligand dissociation from coord-inatively more saturated Pd° complexes871-880 or from reduction of a Pd11 species.353,881 Another route to catalytically active (P—P)Pd fragments is the dissociation of the dinuclear complexes [(//-P—P)Pd]2.882 Complexes [(/r-dcpm)Pd]2 and [(/r-dtbpm)Pd]2 were obtained from the reductive elimination of ethane from dimethylpalladium(II) complexes (dippm = bis(diisopropylphosphino)methane dcpm = bis(dicyclohexylphosphino)methane dcpm = bis(di-t-butylphosphino)methane).883... 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

Having demonstrated that ultrasound can induce ligand dissociation, the initiation of homogeneous catalysis by ultrasound becomes practical. The... 

Either there is a route to skeletal isomerization which can occur without ligand dissociation or else solvolysis of isomer (XX), Scheme IV, can give 30% of the 1-D2 isomer. 

Consequences of unsaturation. Unsaturation in the macrocyclic ring may have major steric and electronic consequences for the nature of the ring. Extensive unsaturation will result in loss of flexibility with a corresponding restriction of the number of possible modes of coordination. Further, loss of flexibility tends to be reflected in an enhanced macrocyclic effect . For example, if the metal ion is contained in the macrocyclic cavity, the loss of flexibility reduces the possible pathways for ligand dissociation and this tends to increase the kinetic stability of the system. As explained in later chapters, enhanced thermodynamic stabilities will usually also result. 

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