Coordination-sphere, metal ion

Sigel, H., and A. Sigel, eds. ENDOR, EPR, and electron spin echo for probing coordination spheres. Metal Ions Biol. Syst. 22 (1987). 

Although the cation-anion interaction of metallocenium ions is very weak, the counteranion is likely to remain in proximity with the metal cation to form a contact ion pair in low-permittivity solvents such as toluene (commonly used in polymerization reactions). If the metal cation allows the counteranion to penetrate into the first coordination sphere, it can form an inner-sphere ion pair (ISIP). When the counteranion is relegated to the second coordinating sphere, the ion pair becomes an outer-sphere ion pair (OSIP), which is more ionic in nature than ISIPs. A schematic representation of the relationship between ISIPs and OSIPs is depicted in Scheme 2. This simple scheme shows us the principal elements that affect the cation-anion interactions (e.g., counteranion (Y ), ancillary ligands (L ), transition metal (M), and alkyl ligand (R)), and the subtle balance between these elements in the dynamic equilibria. 

Furthermore, isomerization of the heptadentate XXXI, coordinated to Ni(II) via transmetallation (91), occurs again aimed at its adaptation to a octahedral coordination sphere around ions with d8 configuration. In the case of the more flexible pentadentate ligands, XXXII and its analogue with three methylene groups (L2) (92), formation of monomeric (with XXXII) and dimeric (with L2), but also of polymeric (with L2) Ni(II) complexes with an octahedral environment around the metal is possible (94). 

This can be achieved by cyclization (crown ethers), by the formation of an intramolecular hydrogen bond (Taddol and Nobin), or by use of a second metal ion to correctly position the chelating atoms within its coordination sphere (metal (salen) complexes). 

The acid strength of proton acid-base pairs (Bronsted acids) is given by the pk value (Table 1.4.2). At the pH that corresponds to the pk, half of the molecules are in the acid form, the other half in the form of the corresponding base. Each pH unit then shifts the ratio by a factor of 10. Phenol, for example, is 50% phenolate and 50% phenol at pH 10, corresponding to a 1 1 ratio at its pk. At pH 7 the ratio of phenohphenolate is then 1000 1. The free energy of acidbase reactions can be directly calculated from the pk values. The pk of water (14 at a concentration of 10 M) is also drastically changed by coordination to metal ions, e.g., to 10 in the coordination sphere of... 

The formation of complexes Ni(L )2 with L 18C6 or R NBr was also proved by spectrophotometry under analysis of UV spectra of absorption of Ni(L )2 and R NBr (18C6) mixtures solutions. At that coordinate with metal ion with preservation of ligand L in internal coordination sphere of complex [90, 92]. Under formation of complexes of Ni(L )2 with U in spite of axial coordination by the fifth coordination place of nickel (II) ion the outer sphere coordination of (H-bonding) with acetylacetonate-ion is also possible. 

Titanium tetrachloride and tin tetrachloride can form complexes that are related in character to both those formed by metal ions and those formed by neutral Lewis acids. Complexation can occur with an increase in the coordination number at the Lewis acid or with displacement of a chloride from the metal coordination sphere. 

Certain metal cations are capable of electrophilic attack on alkenes. Addition is completed when a nucleophile adds to the alkene-cation complex. The nucleophile may be the solvent or a ligand from the metal ion s coordination sphere. 

Transition metal catalysis in liquid/liquid biphasic systems principally requires sufficient solubility and immobilization of the catalysts in the IL phase relative to the extraction phase. Solubilization of metal ions in ILs can be separated into processes, involving the dissolution of simple metal salts (often through coordination with anions from the ionic liquid) and the dissolution of metal coordination complexes, in which the metal coordination sphere remains intact. 

The appearance of the seventh ligand (Xn ) predominantly in the first coordination sphere of the complex or outside of the complex depends on the polarization potential of the alkali metal cation, M+, and on the polarity of the seventh anion, Xn". Increased polarity of the anion favors its entering into the first coordination sphere of the complex ion. 

The specific feature of polymerization as a catalytic reaction is that the composition and structure of the polymer molecule formed show traces of the mechanism of the processes proceeding in the coordination sphere of the transition metal ion to which a growing polymer chain is bound. It offers additional possibilities for studying the intimate mechanism of this heterogeneous catalytic reaction. 

On the basis of these results it seems to the present author that inner and outer complexes can reasonably be assumed for the electron transfer to the diazonium ion, but that an outer-sphere mechanism is more likely for metal complexes with a completely saturated coordination sphere of relatively high stability, such as Fe(CN) (Bagal et al., 1974) or ferrocene (Doyle et al., 1987 a). Romming and Waerstad (1965) isolated the complex obtained from a Sandmeyer reaction of benzenediazonium ions and [Cu B ]- ions. The X-ray structural data for this complex also indicate an outer-sphere complex. 

Coordination Numbers and Radii. In the transition metal ions, the interaction of the ligand orbitals with the d orbitals of the metal ions generally determines the coordination number and geometry of the oordination sphere about the metal. The... 

---