Pentacoordinated compounds, structure

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. 

TABLE 57. Some structural parameters for selected tin and lead pentacoordinate compounds of the type 387 containing a DMC2XY moiety (M = Sn, Pb) with monoanionic X,D-chelating ligand... 

The development of the hypervalency concept was discussed in general. This model was concerned with the group 14 elements . The ability of the tin atom to expand the coordination sphere in organotin compounds was established in the early 1960s, when an oligomeric structure was suggested for trimethyltin fluoride and trimethyltin carboxylates" on the basis of IR data. It should be pointed out that the trimethyltin chloride-pyridine adduct was the first pentacoordinate organotin compound structurally characterized by X-ray diffraction in 1963". ... 

If 2,3-TNPCoCl is reacted with an alkali metal cyanide, the product is not the bridged compound [2,3-TNPCo(CN)] but the pentacoordinated monomer 2,3-TNPCoCN. The same product is obtained by in situ oxidation of 2,3-TNPCo with oxygen in the presence of excess cyanide [14]. Similarly taaCo forms the pentacoordinated compound taaCoCN as shown by its X-ray crystal structure [126]. 

This chapter provides an update on the chemistry of hypervalent phosphorus compounds, bringing in concise form advances in their preparation, application and structural properties, which have been published in 2011. A major portion of this research has been devoted to pentacoordinated compounds whereas relatively little has appeared on hexacoordinate chemistry. 

The crystal structures of [Ag(l,4-dioxane)]AsF69 9 and [Ag2(l,4-oxathiane)](N03)295° have been reported. Two adducts of the 4-nitropyridine A-oxide with silver nitrate have been characterized, one is a mononuclear tetracoordinated compound [Ag(N03)(0NC5H4N02)2] and the other is a dinuclear pentacoordinated species [Ag2(N03)2(/u-0NC5H4N02)2(0NC5H4N02)2] (130).951,952 In the mixed compound /nmv-[RhCl2(py)4]N03-AgN03 exists the dinitroargentate(I) ion in a distorted tetrahedral environment 953... 

The mechanism of these reactions can be described as a nucleophilic attack on a pentacoordinate silicon compound. Supporting this assumption, neutral or anionic hexacoordinated structures have been isolated and their X-ray structures obtained [5]. 

Pentacoordinate phosphorus offers an example of the application of EHT to covalent compounds that do not contain carbon (34). There are two possible high-symmetry structures for PHS, namely, a D3h trigonal bipyramid and a C4v square pyramid. The energies and shapes of tire MO s for each of the two are given in Fig. 26. For the latter, the optimal value of a was found to be 99.8°. Still another structure was considered ... 

Downs et al. reported the synthesis of methylzinc tetrahydridoborate 164 by the two routes shown in Scheme 99.230 The compound is an extremely moisture and oxygen sensitive, colorless, polycrystalline solid, whose solid-state structure was determined by X-ray analysis. Figure 76 shows that 164 consists of helical polymers of alternating methylzinc and tetrahydridoborate ions. The zinc atom is formally pentacoordinate, making this one of the few organozinc compounds with five-coordinate zinc atom. 

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