Coordination Tetracoordination

The possibihty of six-coordinate sihcon species in aqueous solution has been suggested. Raman studies have indicated, however, that monosilicic acid in solution contains a tetracoordinate sihcon species (37). 

Eig. 1. Types of chelates where (1) represents a tetracoordinate metal having the bidentate chelant ethylenediamine and monodentate water (2), a hexacoordinate metal bound to two diethylenetriamines, tridentate chelants (3), a hexacoordinate metal having triethylenetetramine, a tetradentate chelant, and monodentate water and (4), a porphine chelate. The dashed lines iadicate coordinate bonds. 

If there is no other interaction, the reaction proceeds through an acyclic TS and steric factors determine the amount of syn versus anti addition. This is the case with BF3, where the tetracoordinate boron-aldehyde adduct does not offer any free coordination sites for formation of a cyclic TS. Stereoselectivity increases with the steric bulk of the silyl enol ether substituent R1.50... 

Today it is widely accepted that fivefold coordinated silicon plays a key role in the reaction mechanisms of the nucleophilic substitution having a trigonal bipyramidal transition state species which ressemble these transition states can be isolated in some special cases. The structural features fit well to kinetic data and possibly explain the significantly higher reactivity (proved by experimental data) of Si-pentacoordinated compounds compared to their tetracoordinated analoga. 

Analogous considerations apply for tetracoordinate fragments M(CO)4. Fig. 30 shows some of the possible conformations of these fragments. As before, the directional orbitals that develop for particular values of the angle 6 (refer to Fig. 30) allow prediction of possible interaction with donors or of dimerization. Also, the level shifts for variation of 6 in both cases can be calculated, as well as for the squashing mode rearrangement of a tetrahedral into a square-planar coordination. The qualitative confomiational preferences implied by these patterns have been checked, as for the pentacoordinate case, by comprehensive EHT calculations for all dn systems of all conceivable symmetries. 

Complexes formed by tetradentate siderophores involve stepwise complex formation and therefore, have somewhat different equilibria from their hexadentate analogs. Initial chelation will occur with a tetracoordinate FeL complex forming. A subsequent equilibrium then occurs, where the FeL complexes will react in a 2 1 stoichiometry with free ligands in solution to form a single Fe2L3 complex (coordinated water and charges not shown for clarity). 

A tetracoordinated complex (20)4 was actually isolated. Complex 20 in the presence of ethylene forms the coordinated complex 21, as can be seen from H NMR. Complex 21 is a model of the intermediate for the additional reaction to form C6 dienes. The model catalyst had been shown to be a codimerization catalyst under more severe conditions (high temperature), although the rate of reaction was very slow compared to the practical systems. These studies are extremely useful in demonstrating the basic steps of the codimerization reactions taking place on the Ni atom. The catalytic cycle based on these model complexes as visualized by Tolman is summarized in Scheme 7. A more complete scheme taking into consideration by-product formation can be found in Tolman (40). 

The nB NMR chemical shifts (Table 3.13) demonstrate that the boron atom in all known nitronates is tetracoordinated, that is, it is additionally coordinated by... 

Crystals of complex 112 suitable for an X-ray structure determination were obtained on cooling a solution in pentane to — 30 °C. The structure determined is shown in Fig. 7.9. The most remarkable structural feature of 112 is that the gallium center is connected to the zirconium through two different c-carboxylic bridges. One of them contains the cyclo-C6Hg system, which is t]1-bonded to gallium and r 2-coordinated to zirconium. It is noteworthy that carbon atom C-2 is planar tetracoordinate. It is connected to four neighboring atoms in the c-plane, specifically to carbon atoms C-l and C-3 and to both metal centers [175]. 

Protonation of 4b leads to the symmetrically substituted 3b (Scheme 3.2-3) and methylation of 4b at temperatures higher than —60 °C gives 3c (Scheme 3.2-5) [19]. In the latter reaction, 6a can be identified as an intermediate at —80 °C by 13C NMR spectroscopy [19]. Its planar-tetracoordinate carbon atom is strongly de-shielded 3 13 C = 144 ppm) as compared with tetrahedrally-coordinated carbon atoms connected to three boron and one silicon center (d 13C = 70-100 ppm). Computations for the model compounds 6A and 6B give 144 and 104 ppm, re-... 

In the treatment of stereochemical aspects for many chemical problems such as synthetic design representation of tri- and tetracoordinate monocentric configurations by their parity descriptors suffices. The conformational aspect as well as higher coordinate and polycentric configurations can be neglected. Then, it is possible to reduce the CC-matrices to parity vectors Pn whose components +1,0, — 1 represent the configurational features. 

The study of compounds containing pentacoordinate silicon atoms currently represents one of the main areas of research in silicon chemistry. This is evident from the numerous reviews and proceedings published on this topic in recent years.112 Most of the pentacoordinate silicon compounds described in the literature are either salts with A5.S7-silicate anions or neutral silicon complexes with a 4+1 coordination to silicon. This review deals with a completely different class of pentacoordinate silicon compounds zwitterionic A S /-silicatcs. These molecular compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. 

The crystals of both compounds contain pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 88 and 89 are listed in Table XIV. Similar crystal structures were observed for the corresponding oxygen analogs 53 and 77, respectively (see Sections III,D and III, E). As can be seen from the Si-O [1.7600(14)-1.815(4) A], Si-S [2.144(2)-2.1638(9) A], and Si-C distances [1.900(5)-1.906(2) A], the Si02S2C frameworks of 88 and 89 are best described as being built up by five normal covalent bonds rather than a bonding system in the sense of a 4+1 coordination. The Si-S distances are very similar to those observed for tetracoordinate silicon compounds with Si-S bonds. 

In coordinating solvents, such as MeCN and dmso, [Cu(dpa)2]2+ binds two solvent molecules becoming hexacoordinate. In contrast in non-coordinating solvents the complex maintains its tetracoordination. Figure 118 just shows the redox behaviour of the complex in noncoordinating nitromethane solution.175... 

In the C60 fragment all the carbon atoms are tricoordinated, except Cl and C2, which, being coordinated to an oxygen atom of the metal fragment, are tetracoordinated. As shown in Scheme 1, the sphere of carbon atoms, which has an approximate radius of 3.51 A, is composed of 20 six-membered rings fused with 12 five-membered rings with a repetition of the pyracylene building unit. 

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