Labile complex ions

Hexa.cya.no Complexes. Ferrocyanide [13408-63 ] (hexakiscyanoferrate-(4—)), (Fe(CN) ) , is formed by reaction of iron(II) salts with excess aqueous cyanide. The reaction results in the release of 360 kJ/mol (86 kcal/mol) of heat. The thermodynamic stabiUty of the anion accounts for the success of the original method of synthesis, fusing nitrogenous animal residues (blood, horn, hides, etc) with iron and potassium carbonate. Chemical or electrolytic oxidation of the complex ion affords ferricyanide [13408-62-3] (hexakiscyanoferrate(3—)), [Fe(CN)g] , which has a formation constant that is larger by a factor of 10. However, hexakiscyanoferrate(3—) caimot be prepared by direct reaction of iron(III) and cyanide because significant amounts of iron(III) hydroxide also form. Hexacyanoferrate(4—) is quite inert and is nontoxic. In contrast, hexacyanoferrate(3—) is toxic because it is more labile and cyanide dissociates readily. Both complexes Hberate HCN upon addition of acids. 

As with rhodium (and cobalt), introduction of five ammonia molecules is relatively straightforward, but the sixth substitution is difficult, requiring more forcing conditions. One versatile route involves the formation of the pentammine triflate complex ion [Ir(NH3)5(03SCF3)]2+, where the labile triflate group is readily replaced by water, then by a range of anionic ligands [148]. 

The simplest reactions to study, those of coordination complexes with solvent, are used to classify metal ions as labile or inert. Factors affecting metal ion lability include size, charge, electron configuration, and coordination number. Solvents can by classified as to their size, polarity, and the nature of the donor atom. Using the water exchange reaction for the aqua ion [M(H20) ]m+, metal ions are divided by Cotton, Wilkinson, and Gaus7 into four classes ... 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

The reactions of [M(CN)5NO]2 (M = Fe, Ru, Os) with lithium n-butylamide were recently reported (84). Free (E)-ra-butyldiazoate was found as the only product arising from the amide. The intermediate diazoate complex is labile and therefore it is rapidly replaced by other ligands present in the reaction medium. This result provides a strong indication that diazoates (and/or diazoic acids) are intermediates in the reactions of amines with these nitrosyl complexes. This is consistent with a mechanism described in Fig. 18, in which one equivalent of amide adds nucleophilically to the nitrosyl, while a second equivalent rapidly abstracts a proton from the diazoic acid before it decomposes producing a diazonium ion. 

The sequence of Reactions 19, 20, and 21 indicates that substitution on the metal ion by at least one of the substrates is faster than the over-all reaction rate. This can be understood easily for manganese (III) complexes (substitution labile). It is, however, more difficult to rationalize for cobalt (III) unless the rate of cobalt (II)-cobalt (III) exchange is very fast in this system. 

We have seen a quantum leap in progress in the coordination chemistry of copper-dioxygen interactions, resulting in a complete change in thinking about Cu202 structure and related protein chemistry. The problems of copper ion lability, peroxide disproportionation and air/moisture sensitivity have been overcome, and it has been proved that low-molecular-weight Cu 02 or Cu (0)2 complexes can be prepared, and that several different structural types exist. It is apparent that u-r 2 r 2-peroxo coordination is present in oxy-Hc and oxy-Tyr. 

It has been proposed that there may be a close link between the amount of an element available to living matter and the fraction of the total content which is labile (with the lability value being loosely defined as the total, accessible, hydrated ion level). Either the whole or part of the analytical result may be derived from dissociation of labile complex ions or dissolution of moderately soluble compounds. If one or both of these two processes proceed at a relatively slow rate, the magnitude of the lability value becomes time dependent . Conversely, if a complex exchanges ligands fairly rapidly, the amount present in... 

Metal bioavailability is the fraction of the total metal occurring in the soil matrix, which can be taken up by an organism and can react with its metabolic system (Campbell, 1995). Metals can be plant-bioavailable, if they come in contact with plants (physical accessibility) and have a form which can be uptaken by plant roots (chemical accessibility). Soil metals become accessible for humans by ingestion, inhalation and dermal contact. Available forms of PTMs are not necessarily associated with one particular chemical species or a specific soil component. Main soil PTMs pools of different mobility, target organisms and routes of transfer are sketched in Fig. 9.2. The most labile fraction, corresponding to the soluble metal pool, occurs as either free ions or soluble complexed ions and is considered the... 

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