Tetracoordinate complex, stabilization

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

The electrochemical behavior of tetracoordinated Cu(i) complexes (i.e., Cu(dpp)2-based cores) is well established.193,941 The reversible redox potential for the Cu(ii)/ Cu(i) transition is around 0.6-0.7 V versus SCE. This relatively high potential underlines the stability of the 4-coordinate Cu(i) complexes relative to their Cu(n) counterparts. The redox potential of pentacoordinated copper complexes 84 86 is observed in a much more cathodic range. For example, for the 5-coordinate complex Cu(l, dap)2+/+ (dap = 2,9-di-p-anisyl-l, 10-phcnanthrolinc), in which the terpy fragment of the ring is bound to the metal, the redox potential is -0.035 V. This potential shift when going from tetracoordinated to pentacoordinated copper systems is due to the better stabilization of the Cu(ii) state, thanks to the presence in the coordination sphere of live donor atoms. 

In summary, trialkylsilylium ions R3Si+ are stabilized by forming tetracoordinated Si complexes of type III. Pentacoordination at Si will only occur if there are just weak steric interactions between substituents R and solvent S. Mostly, this is parallel to a low internal stability of the silylium ion in question. Accordingly, R2HSi(S)2+ complexes can be observed experimentally while this is not possible for trialkyl silyl cations. [43]... 

Copper-, Ag- and Au-Ge complexes are prepared with the group-IB metal in its +1 oxidation state. The thermal stability and reactivity depend on surrounding both metals with relatively bulky groups, and pure Cu and Ag complexes are isolated only with the metal tetracoordinated. ... 

Thus, it is possible to estimate from Ge NMR the degree of substitution of the Cl atoms by NCS groups when KSCN is added to a suspension of compound 81 in acetone-dg and to observe the formation of the anion 87 in an excess of KSCN. These spectra also indicate that intermolecular exchange between Ge substituents in hexacoordinated derivatives 81-85 proceeds slowly (on the NMR time scale), in contrast to their tetracoordinated derivatives GeCl (NCS)4 , i.e., complex formation stabilizes the GeCl4(NCS)4- molecules. 

Tetracoordinate Boron. A number of tetracoordinate organoboron species that are photo- and electroluminescent, and thus of interest for device applications have been developed.2 Characteristic of complexes (168-170) is the presence of chelating nitrogen ligands such as 8-hydroxyquinoline in (168). The quinolate complexes (168) were reported to show superior stability over more commonly used aluminum complexes. Moreover, their photophysical properties were shown to be tunable through ligand variation. ... 

Olefin complexes of gold(I) can be prepared either by the reaction of tetrachloroau-ric acid with the olefin or directly from the olefin and gold(l) hahde 3 i5 While such species usually have poor stability and decompose in solution at room temperature, the cis-cyclooctene and norbornene complexes of AuCl, obtained from the reaction of [AuCl(CO)] with the corresponding olefin by a CO displacement reaction, have been found to be less prone to decomposition . An (olefin)gold(l) complex featuring a tetracoordinate environment for the gold centre bonded to the olefin has recently been obtained according to the procedure shown in equation 87 . 

In other cases, more sophisticated systems have been designed, for example molecular engines which aim to mimic precise mechanical movements on a molecular scale (Fig. 2) [7]. The principle of movement is based on the variations of stability caused by oxidation or reduction of coordination complexes. In the reduced state the tetracoordination is the most favorable, whereas in the oxide state it is the pentacoordination which is the result of the liberation of a new site of coordination due to the loss of the electron. The intramolecular movement allows the molecule to adopt the most stable configuration by passing from [2+2] to [2+3] coordination by oxidation and conversely from [3+2] to [2+2] coordination by reduction. Thus chemical stability is responsible for the rotation. 

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