Complex anions sizes

Wostyn et al. [61] have reported the molecular nonlinear optical polarizability of lanthanate complexes containing stilbazolium ions. Their experimental results indicate that the hyperpolarizability is independent of the nature of the lanthanide, though the complex anion size is a function of the size of the ligand on the lanthanide cation. Andreu et al. [55] have synthesized a new chiral cyanine dye, 4 -[2-(methoxymethyl)pyrrolidinyl]-l-methylstilbazolium iodide (MPMS + I). They have reported that MPMS +1 exhibits phase-matched SHG with the efficiency of up to 80 times that of urea. 

Careful comparison of Pt-P bond lengths for the series trany-Pt(Pcy3)2X2 (X = H, Cl, Br, I) with those for p-any-Pt(PR3)2X2 (PR3 = PMe3 or PEt3) shows a more definite increase in Pt-P with anion size for the cyclohexyl-phosphine complexes (Table 3.16) believed to be owing to intermolecular X- H and X- C non-bonded interactions arising from overcrowding [151]. 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

In the presence of deliberately added electron-acceptors, X, e.g., tetracyanoethylene, the anion forms a complex XA Because of the greater size and lower charge-density of this complex anion, the fraction of unpaired cations will be increased, with consequent effects on rate, DP, copolymerisation ratios, tacticity, etc., of the polymers. 

The complexation of anionic species by tetra-bridged phosphorylated cavitands concerns mainly the work of Puddephatt et al. who described the selective complexation of halides by the tetra-copper and tetra-silver complexes of 2 (see Scheme 17). The complexes are size selective hosts for halide anions and it was demonstrated that in the copper complex, iodide is preferred over chloride. Iodide is large enough to bridge the four copper atoms but chloride is too small and can coordinate only to three of them to form the [2-Cu4(yU-Cl)4(yU3-Cl)] complex so that in a mixed iodide-chloride complex, iodide is preferentially encapsulated inside the cavity. In the [2-Ag4(//-Cl)4(yU4-Cl)] silver complex, the larger size of the Ag(I) atom allowed the inner chloride atom to bind with the four silver atoms. The X-ray crystal structure of the complexes revealed that one Y halide ion is encapsulated in the center of the cavity and bound to 3 copper atoms in [2-Cu4(//-Cl)4(//3-Cl)] (Y=C1) [45] or to 4 copper atoms in [2-Cu4(/U-Cl)4(/U4-I)] (Y=I) and to 4 silver atoms in [2-Ag4(/i-Cl)4(/i4-Cl)] [47]. NMR studies in solution of the inclusion process showed that multiple coordination types take place in the supramolecular complexes. 

The electrostatic cation-anion interactions will depend on the properties of the anion its charge, its size, its shape, its polarizability. Large anions lead to weaker interactions because of larger cation-anion distances ion-paired complexes of divalent AEC s will be much more destabilized by an increase in anionic size than AC complexes ... 

The protonated polyaza macrocycles (27-6H+ and 28-8H+) also complex metal hexacyanide anions (81CC1067). Both Fe(CN)64- and Ru(CN)64- form 1 1 complexes which are more difficult to oxidize than the uncomplexed anion. The shifts in anodic oxidation potential are independent of the M(CN)64- species involved but vary with the size of the macrocycle (+130 mV for 27-6H+ and +165 mV for 28-8H+). This suggests that the redox potential of complexed anions is controlled by electrostatic effects due to the charge of the surrounding macrocycle. 

As the ratio of organic to aqueous in a mixed solvent is increased, the concentration of water molecules around the cation is reduced, decreasing the size of the hydrated cation. One consequence of this is that metal complex anions will be formed at lower anion concentrations than in pure water. Moreover, because the forces that bind the hydration cloud depend on the charge density of the cation, selective destruction of the hydration cloud starts at lower organic aqueous ratios for larger cations.399 As a result, large cations will penetrate a cation exchanger more easily, and differences in the distribution coefficients of elements of different size will be enhanced. 

In analogous fashion, the ruthenium- or iridium-based bimetallic and trimetallic CTV complexes such as 54-57 also make excellent anion hosts. The CTV cavity, however, is wider and more shallow and hence, the BF anion is too small to be effectively bound. This is evidenced by the crystal structures of the BF J salts of 56 and the iridium analogue 57 in which the anion within the cavity is either extremely disordered or is situated to one side of the cavity. We have found that host 54 exhibits a pronounced selectivity for TcO in nonaqueous solutions, presumably as a consequence of the wide cavity in conjunction with the larger anion size.104,109 Indeed, the X-ray crystal structure of the mixed ReO CFjSOj salt shows the... 

Slow exchange may lead to extremely broad polydispersities and often to polymodal MWDs. It is recommended to study the evolution of molecular weights with conversion and especially the proportion and position of various peaks in the MWD by size-exclusion chromatography (SEC). Use of scavengers is helpful in the identification of the origin of peaks in SEC (MWD) traces. For example, salts with common ions suppress free ions and reduce the intensity of peaks formed by free ions. Hindered pyridines trap protons and reduce peaks resulting from protonic initiation, especially in systems with adventitious moisture. Apparently, stability of complex anions MtX +, and MtX OH can be different and slow exchange may lead to polymodal MWD. 

The stability of fluorine-containing complex anions is apparently related to steric reasons (the smallest size of F atom) and to the highest electronegativity of fluorine. 

The results of studies for complexes formed between polyammonium macrocycles and transition metal complex anions indicate that cation anion electrostatic attraction is a crucial factor in complexation reactions and serves to regulate the stoichiometry of the complexes formed. Hydrogenbonding, size, and conformational factors also play major roles. Anions can be incorporated in or out of the ring. Two illustrative examples are metal ion complexes with the octaprotonated macrocycle Hg[30]aneNio (46). In the complex with Co(CN)6 , the anion hes outside the macrocycle. The PdCLi complex is a true inclusion situation, however, in which the PdCU is situated along the minor axis of the macrocychc cavity, and the Cl atoms are out of the frame, forming strong hydrogen bonds with the polyammonium... 

The mobility of uranium in soil and its vertical transport (leaching) to groundwater depend on properties of the soil such as pH, oxidation-reduction potential, concentration of complexing anions, porosity of the soil, soil particle size, and sorption properties, as well as the amount of water available (Allard et al. 

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