Chelating ligands, hexacoordinate

Hexacoordinated silicon takes benefit from the favorable octahedral arrangement of ligands around a coordination center. As expected, negatively charged substituents are superior in stabilizing high silicon coordination numbers. Most recently however, compounds with chelate ligands, mono anionic derivatives and compounds with more electropositive donors have been added to the series of penta- and hexacoordinated silicon species. 

Complexation of the well-known dithiocarbazate chelate ligand (DTCA ) with Al3+, Sn2+, and Sn4+ ions leads to the formation of hexacoordinated complexes with 1 1 (M + DTCA) ratio, which were investigated for antimicrobial activity <2001SRI115>. These compounds, however, were found to be thermally unstable. 

Complexes 521865, 5221132 and 5231132 are examples of hexacoordinate diorganotin compounds bearing dianionic tridentate 0,N,0-chelating ligands. 

The oxo moiety in these complexes exerts a strong trans effect in both the structural and kinetic senses. In the case of dichloro[hydrotris(l-pyrazolyl)borato]oxotech-netate(V)93 hexacoordination is forced by the chelating ligand however, the three pyrazole nitrogens are not equivalent. The Tc-N bond distances are 2.086 A, 2.088 A and 2.259 A, with the latter distance belonging to the nitrogen trans to the oxygen. 

Dichloro neutral bis-chelates of hexacoordinate silicon were obtained by two methods directly (35) by transsilylation of Af-dimethylamino-O-(trimethylsilyl)acylimidates (1) with tetrachlorosilane, or by rearrangement as described in the previous section (70). The presence of two chloro ligands was utilized for further substitution, to form hexacoordinate silicon tris-chelates, by reaction with bis(trimethylsilyl)-precursors 71 and 72, as shown in Eqs. (31) and (32). The products (73, 74, and 75), are hexacoordinate neutral silicon tris-chelates, and 75 is the first reported zwitterionic tris-chelate with three different chelate rings.73... 

Summary Our investigations on the coordination behavior of ethylene-ACV -bis(2-oxy-4-methoxybenzophenoneiminate) 1, a tetradentate chelating ligand of the salen type, toward diorgano-substituted silicon atoms led to the syntheses and X-ray structure analyses of novel hexacoordinate silicon complexes. The first X-ray structures of a metal-free hexacoordinate dimethyl silane and of a hexacoordinate cw-configurated silicon complex with a salen type ligand are presented. 

Hypercoordinate silicon complexes with tetradentate (O, N, N, 0)-chelating ligands of the salen type are expected to exhibit unusual chemical and physical properties because of the higher coordination number of the silicon atom [1,2]. Therefore, several attempts were made to synthesize such compounds [2, 3]. Starting from easily available silicon compounds such as SiCU or other chlorosilanes, conversion with salen type ligands mostly yielded complexes with a hexacoordinate [2, 3] and, in some cases, pentacoordinate silicon atom [4]. Unfortunately, there are only a few examples where the coordination geometry has been confirmed by X-ray structure analysis [2, 4]. 

It should be noted here that Corriu and co-workers have reported on organosilicon compounds containing C,N-chelating ligands in which the silicon atom is penta- or hexacoordinate (55-58). Such organosilicon compounds are models for the intermediates proposed for the activation of enoxysilanes by F leading to cross-aldolizations or Michael reactions (59). 

Fig. 19. Triorganotin halides containing two C,N-chelating ligands in which the tin atom may be regarded as hexacoordinate.

The tin atom in monoorganotin trihalides in which a C,Y-chelating ligand is present have a trigonal bipyramidal coordination geometry in the solid state. However, in solution when external donor molecules are also present, hexacoordinate species may be formed. 

The synthesis of four-coordinate compounds of d8 metals is much simpler than for other ions, however, some effective strategies can be used for this proposal and one of them is the protonation of the leaving group. Thus, the strategy for synthesis of tetracoordinate complexes starting from hexacoordinate precursors involves the dissociation of the coordinate chelate ligand by an acid-catalyzed dissociation (Fig. 14.5). 

The molecular structure of SiF4-2py (59) shows a centrosymmetric octahedral geometry at silicon with the pyridine molecules trans to each other and an Si-N distance of 1.93 Hexacoordinate geometry may also occur when using a chelating ligand such as bipyridine as in complex (60), where the N Si distance averages 1.977... 

The chemistry of heterometallic alkoxides involving a common chelating ligand, Al(OPr )4 finds a close parallelism in an early suggestion by Bradley that tetrameric aluminium isopropoxide Al(OPr )3 4 may be represented by Al (/r-OPr )2Al(OPr )2 3 in which the central aluminium is hexacoordinated by being ligated with three bidentate Al(OPr )4 figands. This unusual structure of... 

Equilibria between hexa- and pentacoordinated sUicon complexes have been examined carefully by Kost et al. Exemplaiily the following works might be cited here Neutral hexacoordinated siUcon complexes derived from hydrazide chelating ligands with imino-donor groups form pentacoordinated Si complexes in the course of ionic dissociation reactions. Such complexes were shown to undergo facile intramolecular aldol-type condensation [176]. In a related system... 

A number of hexacoordinated silicon complexes have been prepared by using benzamidinato ligands mainly in combination with other chelating ligands [154,262]. Examples are compound 72 (Scheme 15) and the compound classes 115 and 116. 

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