Complexation stoichiometry

A Ca2+-specific fluorescent chemosensor 25 in aqueous buffer signals Ca2+ via a decrease in fluorescence intensity, whereas excess of Mg2+ ions has no effect on the emission [85]. This probe has limited solubility in aqueous solution after binding to Ca2+. A Zn2+ sensitive probe 26 showing different fluorescence responses depending on the complexation stoichiometry is described in [86],... 

The gallium and indium complexes of novel bis(thiosemicarbazones) have been investigated.71-73 Initial publications indicated a more complex stoichiometry where X-ray crystal structures suggested either dinuclear or trinuclear complexes. Whereas these complexes are quite interesting, they are less likely to result in useful nuclear medicine radiopharmaceuticals. The bis(thiosemi-carbazones) are quite useful as ligands for copper, and are discussed in more detail vide infra). 

Table 1 Association (k+) and dissociation rate constants (k ) for the binding of ethidium bromide to DNA assuming a 1 1 complexation stoichiometry...

In principle, it appears possible to distinguish between the formation of monodentate and bidentate surface complexes if the stoichiometry of the H+ release is know. A mean surface complex stoichiometry can be formulated... 

Complexation of dendritic ligands 1 and 2 with lanthanide ions (Nd3+, Eu3+, Gd3+, Tb3+, Dy3+) [17f] leads to results qualitatively similar to those obtained upon Zn2+ complexation (see above) an increase in the monomer naphthalene emission band at 337 nm and a complete disappearance of the exciplex band at 480 nm. However, the complex stoichiometry is different. Emission data were best fitted considering the formation of 1 3 and 1 2 (metal/ligand) complexes (log f 1 2 = 14.1 and log [ivi = 20.0) in the case of 1 and a 1 3 (metal/ligand) complex (log / 1 3 = 20.3) for compound 2. Therefore, at low metal ion concentration, only the [M(2)3]3+ species is present, as also demonstrated by NMR titration. It is likely that in this complex... 

Proton nmr titration experiments of [26] and [27] with KPF6 in acetonitrile revealed that in solution both compounds form 1 1 intramolecular sandwich complexes with the potassium cation. A number of alkyl-, vinyl- and azo-linked bis(benzo-15-crown-5) ligands are well known to exhibit this mode of K+ coordination. In the case of [26], a solid-state potassium complex was isolated whose elemental analysis and fast-atom bombardment mass spectrum ([26] K+ = 1083 complex ion) was in agreement with 1 1 complex stoichiometry (Fig. 20). 

A SELECTION OF MORE COMMON INTERMETALLIC PROTOTYPES HAVING MORE COMPLEX STOICHIOMETRIES AND STRUCTURES... 

Figure 2 shows that for a polymer couple, even for a large excess of polybase, the fraction of carboxylic groups actually complexed (given by the compelxation degree 0) is always smaller than one and strongly depends on a (or on p, see Figure 3). This corresponds to a variable mean stoichiometry in contradiction with most of the previous papers, where a mean complex stoichiometry close to 1 1 is proposed (5-10). but in agreement with Morawetz s results (16).

The guest cations hitherto examined cover broadly uni- to trivalent and inorganic to organic ions that include alkali, alkaline earth, heavy and transition metal ions, as well as (ar)alkyl ammonium and diazonium ions. As to the complex stoichiometry between cation and ligand, both 1 1 stoichiometric and 1 2 sandwich complexes are analyzed. The solvent systems employed also vary widely from protic and aprotic homogeneous phase to binary-phase solvent extraction. 

Change-transfer complexes of solute-alcohol stoichiometry 1 2 have been reported by Walker, Bednar, and Lumry3 for indole and certain methyl derivatives (M) in mixtures of associating solvents n-butanol and methanol (Q) with n-pentane these authors introduced the term exciplex to describe the emitter of the red-shifted structureless fluorescence band which increases in intensity with the alcohol content of the mixed solvent. The shift of the exciplex band to longer wavelengths as the solvent polarity is increased, described by Eq. (15), confirms the dipolar nature of the complex that must have the structure M+Q2. No emission corresponding to the 1 1 complex is observed in these systems which indicates (but does not prove) that the photo-association involves the alcohol dimer. The complex stoichiometry M+Q determined from (Eqs. 9, 10, and 12)... 

Complexes are also formed in certain instances between neutral molecules and macrocyclic receptors. Neutral molecules which form such complexes for the most part contain polar O—H, N—H or C—H bonds, and hydrogen bonding interactions are responsible for the solid state structural characteristics of these complexes. For many of these complexes, stoichiometries range considerably, from 1 1 to 1 6 host guest, and include a variety of odd ratios such as 3 2, 2 7, etc. Structural results for these complexes indicate them not to be of the inclusion type in a majority of cases. Thus, discussion in this subsection will be limited to a general overview. A more complete review of neutral molecule complexation can be obtained elsewhere.21... 

Complex stoichiometry problems should be worked slowly and carefully, one step at a time. When solving a problem that deals with limiting reactants, the idea is to find how many moles of all reactants are actually present and then compare the mole ratios of those actual amounts to the mole ratios required by the balanced equation. That comparison will identify the reactant there is too much of (the excess reactant) and the reactant there is too little of (the limiting reactant). 

Actually, this seemingly simple reaction is, from a mechanistic point of view, a rather complicated multicomponent reaction that involves two / -toluidine A and three formaldehyde B molecules (41JA832). Although TB 1 (A2B3) is the main product, other heterocycles have also been isolated (Scheme 2), some with more complex stoichiometries such as A3B4. 

Ligand- macromolecule complex Stoichiometry of complex Kinetics of binding Location of interacting sites Orientation of bound ligand Structure of complex Dynamics of complex Chemical shift titration Line width, titration analysis HSQC, isotope editing NOE docking 3D/4D NMR Relaxation time measurements... 

The intermolecular (interionic) distances must be regular. This "mixed valency" requires that there be only one crystallographically unique molecular site, which must share its partial valency with the nearest neighbor sites along the stack. The many "complex stoichiometry" TCNQ salts—for example, Cs2(TCNQ)32 or triethylammonium(TCNQ)2-, which exhibit "trimeric" or "tetrameric" units of several crystallographically distinct TCNQ molecules and TCNQ- anions held at van der Waals separations—do not conduct well. 

The third feature of dithiolene chemistry that attracted early attention was the chemical and electrochemical one-electron redox reactivity of the complexes, which allowed a given complex stoichiometry (M/L ratio) to be isolated with several different charges (i.e., different states of oxidation, albeit not necessarily different oxidation states of the metal). Chapter 5 deals with the electrochemical and chemical reactivity of dithiolene complexes, wherein it is seen that the chemical reactivity goes beyond simple redox reactivity and includes reactions that are often ligand, rather than metal, based. 

---