Chelating ligands, hexacoordinate complexes

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. 

In this series the aromatic ligands catechol and tropolone afford chelates whose chemical shift is a function of the number of oxygen atoms around silicon and the nature of the non-chelating group attached to silicon. Thus the cationic and anionic hexacoordinate complexes [41], [51], and [52] have essentially the same chemical shifts whether tropolone or catechol is the ligand. 

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]. 

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... 

A convenient method for the preparation of neutral bis(N->Si) hexacoordinate silicon complexes has been developed and reported recently, consisting of ligand exchange between a polychlorosilane (1) and 0-trimethylsilyl derivatives of hydrazides (2, Eq. 1) [2]. An attempt to utilize this synthetic route for the preparation of isomeric 0->Si coordinated chelates did not lead to the expected hexacoordinate complexes, but to ionic siliconium chloride salts stabilized by two (O—>Si) dative bonds (5, Eq. 2) [3]. 

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. 

A large number of papers have been published in the last 5 years dealing with penta- and hexacoordinated silicon complexes stabilized by tridentate 0,N,0-, 0,N,S-, N,N,S-, and 0,N,N-chelate ligands. A substantial part of these were produced by Tacke et al. [189, 190, 193, 194, 268-272]. Most of the tridentate ligand systems used in their investigations are Schiff bases of aromatic o-hydroxy-or o-mercaptoaniline derivatives. These were used to prepare penta- and hexacoordinated silicon complexes 124-130. 

---