Weak complexes

It requires a certain flexibility of mind to accept the proposal of using the same THF as extraction solvent in some cases. Ue discovered this possibility, when we tried to remove this solvent from carboxylic acids in a water-pump, vacuum it appeared difficult to remove the last traces of this solvent, even when heating at 70-80°C in a vacuum of 10-15 mmHg was applied. It seemed that there is some weak complexation. This led us to the idea of using THF for the extraction of carboxylic acids from the aqueous phase (after saturation with... 

The strength of this bonding depends on the kind of ether Simple ethers form relatively weak complexes with metal ions but Charles J Pedersen of Du Pont discovered that cer tain polyethers form much more stable complexes with metal ions than do simple ethers Pedersen prepared a series of macrocyclic polyethers cyclic compounds contain mg four or more oxygens m a ring of 12 or more atoms He called these compounds crown ethers, because their molecular models resemble crowns Systematic nomencla ture of crown ethers is somewhat cumbersome and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms m the ring and is followed by the number of oxygen atoms... 

The solubility of the actinides in the organic phase, the right-hand side of equation 3, is achieved by the weak complexes that tri- -butyl phosphate... 

Complex Ion Formation. Phosphates form water-soluble complex ions with metallic cations, a phenomenon commonly called sequestration. In contrast to many complexing agents, polyphosphates are nonspecific and form soluble, charged complexes with virtually all metallic cations. Alkali metals are weakly complexed, but alkaline-earth and transition metals form more strongly associated complexes (eg, eq. 16). Quaternary ammonium ions are complexed Htde if at all because of their low charge density. The amount of metal ion that can be sequestered by polyphosphates generally increases... 

The O or S atoms in P=0 and P=S groups may act as electron donors although these groups form relatively weak complexes with electron acceptor compounds such as nonpolarizable, more electropositive (ie, hard) acids, including protons (14). Use is made of this property in the recovery of uranium from wet-process phosphoric acid by extractants such as trioctylphosphine oxide [78-50-2] and di(2-ethylhexyl) hydrogen phosphate [298-07-7]. 

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

The strength of this bonding depends on the kind of ether. Simple ethers fonn relatively weak complexes with metal ions, but Charles J. Pedersen of Du Pont discovered that certain polyethers fonn much more stable complexes with metal ions than do simple ethers. 

The rightmost structure is a weak complex of the products (having a binding energy of 1 kcal mol ), and for our purposes maybe construed as the reaction end point. 

Exciting developments have occurred in the coordination chemistry of the alkali metals during the last few years that have completely rejuvenated what appeared to be a largely predictable and worked-out area of chemistry. Conventional beliefs had reinforced the predominant impression of very weak coordinating ability, and had rationalized this in terms of the relatively large size and low charge of the cations M+. On this view, stability of coordination complexes should diminish in the sequence Li>Na>K>Rb> Cs, and this is frequently observed, though the reverse sequence is also known for the formation constants of, for example, the weak complexes with sulfate, peroxosulfate, thiosulfate and the hexacyanoferrates in aqueous solutions. 

Examine the geometries (in particular, CN bond distances) of methyl diazonium, tert-butyl diazonium and phenyl diazonium ions. Which, if any, of these ions is best described as a weak complex between a cation and N2 Which is furthest away from this description Is your result consistent with the observed reactivity patterns Explain. 

Step through the sequence of structures representing dissociation oiketene to methylene and carbon monoxide. Plot energy (vertical axis) vs. carbon-carbon bond distance (horizontal axis). Would you describe ketene as a weak complex between singlet methylene and carbon monoxide Explain. (A table of CC and CO bond lengths is found at left.) Is there an energy barrier to the dissociation ... 

RuCl2(PPh3)2 reacts with 4-R2P-dibenzothiophene (R = Ph, p-Tol) and forms 303, in which the dibenzothiophene ligand is coordinated to ruthenium via the phosphorus and sulfur atoms [84JA5379, 87JOM(318)409]. The donor ability of the sulfur atom is relatively weak. Complex 303 (R = Ph) is able to add carbon monoxide and yield the monocarbonyl adduct. 

If weak complexes present in solution consist of dimers alone (Fig. 7), association patterns of homodimers (11 and 13) are of the head-to-tail type, that of heterodimer (72) being of the head-to-head type, since the two acylurea bonds in 1 and 2 extend... 

This colour change can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni, and the Pt metals. To maintain the pH constant (ca 10) a buffer mixture is added, and most of the above metals must be kept in solution with the aid of a weak complexing reagent such as ammonia or tartrate. The cations of Cu, Co, Ni, Al, Fe(III), Ti(IV), and certain of the Pt metals form such stable indicator complexes that the dyestuff can no longer be liberated by adding EDTA direct titration of these ions using solochrome black as indicator is therefore impracticable, and the metallic ions are said to block the indicator. However, with Cu, Co, Ni, and Al a back-titration can be carried out, for the rate of reaction of their EDTA complexes with the indicator is extremely slow and it is possible to titrate the excess of EDTA with standard zinc or magnesium ion solution. 

Diazoalkanes and related compounds are not suitable guests for the types of hosts discussed above. Very weak complexation was found with diazodicyanoimidazole (2.53 Sheppard et al., 1979) in which the mesomeric zwitterionic structure with a formal diazonio group (see Secs. 2.6 and 6.2) is dominant. However, no complexation was found for another compound with a formal diazonio group, the ben-zothiazol-azidinium salt 2.50 (Szele and Zollinger, 1982). 

There is, however, another possible explanation. For relatively weak complexes, as in these cases, a complex other than one of the insertion type may form in solution, for example a charge-transfer complex. An early observation which may indicate the formation of other types of complexes was reported by Bartsch and Juri (1980), but not interpreted the dediazoniation rate for 4-tert-butylbenzenediazonium tetra-fluoroborate in 1,2-dichloroethane decreases by 12% in the presence of one equivalent of 15-crown-5, a host compound which does not form insertion complexes. Kuokkanen and Virtanen (1979) also observed some stabilization towards dediazoniation of 2-toluenediazonium ion by 18-crown-6, even though, for steric reasons, an insertion-type complex is hardly possible in this case. 

The two most soluble (in H2O) compounds of Hg(I) are Hg2(N03>2 2 H2O and Hg2(C104)2 4 HjO, and these are used as sources of aq Hg2 ions for the preparation of less soluble Hg(I) compounds. They are both made by redox reactions involving Hg(0) and Hg(II). Solutions of these two salts do not disproportionate because the anions [NO ]" and [CI04] are poor complexing ligands and form only weak complexes with Hg(II). Sparingly soluble Hg(I) derivatives can be formed by... 

Given the zwitterionic natnre of single carbenes, the possibility exists for coordinating solvents such as ethers or aromatic compounds to associate weakly with the empty p-orbital of the carbene. Several experimental stndies have revealed dramatic effects of dioxane or aromatic solvents on prodnct distribntions of carbene reactions. Computational evidence has also been reported for carbene-benzene complexes. Indeed, picosecond optical grating calorimetry stndies have indicated that singlet methylene and benzene form a weak complex with a dissociation energy of 8.7kcal/mol. ... 

Fortunately, for this solvent, the electron-capture centres give very broad e.s.r. features at 77 K, and hence the spectra for S + cations are readily distinguished. We know of no instance in which S + cations are not formed provided the ionization potential of S is less than that of the solvent. There are two complicating factors, one is unimolecular break-down or rearrangement of the radical cations, and the other is weak complexation with a solvent molecule. The latter is readily detected because specific interaction with one chlorine or one fluorine nucleus occurs, and the resulting hyperfine features are usually well-defined. 

Since this original work many new crowns have been synthesized and their complex formation has been very extensively studied. Complexes with many other ions (including weak complexes with several transition ions) have been characterized. Crystalline 1 1 (metaldigand), 1 2, and2 3 complexes as well as species of other stoichiometry have all been isolated. 

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