Transition metals second coordination sphere

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. 

The binding of complex anions of transition metals such as the hexacyanides M(CN)6m yields second-coordination-sphere complexes, supercomplexes, [3.13a] and affects markedly their electrochemical [3.21, 3.22] and photochemical [3.23] properties. 

Molecules in the second coordination sphere, that is, solvent or partners of an ion pair, can transfer charge to the metal ion of a coordination complex in an optically induced electronic transition. The excited states are, respectively, known as solvent to metal charge transfer (SMCT) and ion pair charge transfer (IPCT) excited states. A typical example are the ion pairs, [Cora(NH3)6]3 +, X-, formed between [Coln(NH3)6]3 + and halide and pseudohalide ions X. The UV-Vis spectrum of ion... 

Gagn6 et al. developed polymer active sites that additionally contained a receptor (recognition sites) displayed in the outer sphere of the metal center (reactive site). The Suzuki reaction of /)-bromoanisole with phenyl boronic acid and the allylation of dimethyl malonate with allyl acetate were both chosen to assess the presence and/or effect of the crown-ether in crown-ether-molecular imprinting polymer-palladium complex. The results showed that molecular imprinting can be used to functionalize the second-coordination sphere of a transition metal complex and subsequently affect its catalytic behavior. 

Catalysis by transition metal complexes and metaUoenzymes involves a sequence of ligand exchange reactions. Such a reaction can he viewed as an interchange of an inner sphere and outer sphere ligand. It is argued therefore that the structure of the second coordination sphere is of direct relevance to catalysis. A method of studying second coordination sphere structure based on dipolar NMR shifts in paramagnetic complexes is discussed. Even in weakly bound complexes, there is a definite preferred structure for the complex which may or may not be favorable for a subsequent substitution reaction. 

There have been attempts to improve synthetic catalysts by creating an artificial second coordination sphere. These efforts have included covalently attaching a metal center to a macromolecule or by incorporating the transition metals into hosts, such as zeolites [4]. Not surprisingly, there has also been considerable activity focused on the development of catalytically active metal complexes with imprinted polymers. The polymerization process develops a matrix around the active site that can... 

The UV-Vis spectral data do not contradict to corresponding IR data suggesting the Fe presence in the second coordination sphere of Si. The UV-Vis electron spectrum of the 15 min activated mixture is characterized by absorption near 11000 cm caused by Fe d-d transition and strong absorption in the region of 30000-40000 cm caused by charge transfer metal-ligand (Figure 26). There is also a weak band at 21200 caused by Fe d-d transition. The band near 11000 cm" reflects the presence of Fe in octahedral coordination. However weak bands at 21200 cm" can be caused by d-d transitions of Fe in tetrahedral coordination [74-76]. 

The simplest of the ir-bondcd Re-C compounds is the green, paramagnetic, crystalline, therm ly unstable ReMen, w ich, after WMe, was only the second hexamethyl transition metal compound to be synthe zed 11976). It reacts with LiMe to give the unstable, pyrophoric, Lii[ReMe(,, which has a square-antiprismatic structure, and incorporation of oxygen into the coordination sphere greatly H reases the stability, wit e,ss Re CMe, which is thermally stable up to 200 C, and Re "0[Pg.1068]

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