Pentacoordinated anions

FIGURE 40. Optimized geometries of D31, MII5 anions calculated at QCISD/6-311++G(d,p) for M = Si, QCISD/TZ2P with two sets of diffuse s and p functions for M = Ge and Sn and QCISD/RECP for M = Pb. Bond lengths in pm data from Reference 168 

This impressive progress in theoretical methodology resulted in intensive computational work, which is summerized in this review and in its nearly 650 references, and in a much deeper understanding of the periodic trends, major as well as subtle, expected as one moves down along the group from carbon through silicon and germanium to tin and lead. 

The relatively short experience in this field has taught us that the future lies in the close collaboration and joint research of theoreticians and experimentalists. The models that can 

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In the case of anionic systems, following the ideas of Diinitz [6], crystal structure (C) is a good model of an intermediate in which a nucleophile attacks a silicon atom included in a pentacoordinated anionic structure. Such an intermediate occurs during the nucleophilic displacement illustrated in Scheme 3. 

Alternatively, some conclusions can be derived from the relative reactivities of car-banions. For example, DePuy and colleagues13 made use of a clever method involving reactions of silanes with hydroxide ion to deduce acidities of such weak acids as alkanes and ethylene. The silane reacts with hydroxide ion to form a pentacoordinate anion that ejects a carbanion held as a complex with the hydroxysilane rapid proton transfer gives the stable silanoxide ion and the carbon acid (equation 5). 

The constant potential electrolysis of tetraaikylsiianes in the presence of Et4NH3F4 at 2.3 V results in the cleavage of the C-Si bond and the formation of the corresponding fluorosilanes [21], Presumably, the first step involves the formation of the pentacoordinate anionic species (RR 3SiF ) which is oxidized... 

Pentacoordinated anionic silicon adducts had earlier been demonstrated to be intermediates in anionic gas-phase reactions G. Angelini, C. E. Johnson and J. I. Brauman, Int. J. Mass Spectrom. IonProc., 109, 1 (1991). 

A theoretical comparison between the structures and properties of cyclic pentacoordinate anionic pentaoxysilicates and phosphoranes is described237. The results are compared with NMR and X-ray crystallographic data. 

The mechanism involves the preferential attack of the hydride on the less hindered silicon with formation of a pentacoordinated anionic species which collapses to give an a-silyl carbanion intermediate and SiH4 gas. The a-silyl carbanion may then take up a hydrogen from the hydrogen source (e.g., silane or solvent) to yield 7 or may lose... 

Formation of a pentacoordinate 1,2-oxastannetanide by a base-induced rearrangement of a bis(/3-hydroxyalkyl)-stannane has also been reported <1997PS513>. Synthesis of pentacoordinate anionic oxasiletanides, oxagermeta-nides, and oxastannetanides has been reviewed (Scheme 17) <19980M367, 2000JOM256, 2002CSR195>. 

TABLE 9. X-ray data for pentacoordinate anions R3SnX2 with ligands containing N or O coordinating atoms... 

TABLE 12. Selected structural parameters for bridged pentacoordinate anionic stannatesa... 

Various monocyclic and bicyclic pentacoordinate anionic germanium and tin complexes are known which possess bidentate dianionic oxo and related ligands, such as aliphatic or aromatic 1,2-diols, a-hydroxy carboxylic acids and their thio analogues (for early references see References 191, 307-309). Anionic mono- and bis-chelated germanium and... 

Holmes and his coworkers described a number of pentacoordinated anionic germanium complexes containing a spirocyclic framework with methyl, phenyl, halogen or hydroxyl ligand at the acyclic site307,332,333. Reaction of an organogermanium trichloride with a catechol or thiocatechol derivative in the presence of triethylamine, followed by a metathetical exchange, led to desired products as illustrated by the formation of the phenyl-substituted derivatives 90 and 91 (equation 16)333. 

Tacke and his coworkers have recently reported the synthesis and structural characterization of a series of pentacoordinated anionic germanium complexes based on the Ge04C ligand framework340-344. The route to these compounds involves the reaction of (chloromethyl)trimethoxygermane with a secondary amine in the presence of triethy-lamine, followed by an exchange with two equivalents of bidentate ligands, such as aromatic 1,2-diols or a-hydroxycarboxylic acids, as shown for the synthesis of 114340 (equations 22 and 23). 

Holmes and his coworkers described a number of pentacoordinated anionic germanium complexes containing a spirocyclic framework with methyl, phenyl, halogen or hydroxyl ligand at the acyclic Reaction of an organogermanium trichloride with... 

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