Trigonal pyramidal coordination

Most tin(II) compounds display structures with a trigonal pyramidal coordination. This is of course to be expected as the tin atom is in the first place electrophilic in order to complete its outer electron configuration (cf. Chapter 5 and 6). To illustrate the resemblance of this geometry between ionic and molecular compounds, the structure of NH4SnF3 (5) 31) is compared with that of the cage compound (Me3CN)3(Me3A10)Sn4 (6) 32). The coordination sphere of the tin atom is the same in 5 and 6 (for the complete structure of 6 see Sect. 6.5) ... 

Blue copper proteins in their oxidized form contain a Cu2+ ion in the active site. The copper atom has a rather unusual tetra-hedral/trigonal pyramidal coordination formed by two histidine residues, a cysteine and a methionine residue. One of the models of plastocyanin used in our computational studies (160) is pictured in Fig. 7. Among the four proteins, the active sites differ in the distance of the sulfur atoms from the Cu center and the distortion from an approximately trigonal pyramidal to a more tetrahedral structure in the order azurin, plastocyanin, and NiR. This unusual geometrical arrangement of the active site leads to it having a number of novel electronic properties (26). 

Among the first to be described was the iminoindane complex [ MeIn(NC6H4F-4)-(thf) 4] prepared as shown in Scheme 8.1. The framework consists of a distorted cube with In and N atoms at alternating corners (In—N bond lengths are between 2.19 and 2.41 A). The distorted trigonal pyramidal coordination geometry at each indium atom is... 

Obviously, this is a result of the incorporation of one Ca atom into the center of the cage which, has fairly short Ca-Si distances of 288 pm although two Si atoms are pushed outwards. The central barrelane unit in the binary compmmd is again trigonal pyramidally coordinated by calcium, but without a central Ca atom. 

It can also be seen from Figure 11 that in every case the trigonal pyramidal coordination around tellurium is expanded into an AX3Y3E octahedron by Te- - -Cl secondary contacts of 3.219 3.716 A. In the case of PhsTeCl one of the three secondary contacts is to the center of a phenyl ring of the adjacent PhsTe+ cation. ... 

The different coordinations of the H2O molecules observed in solid hydrates are due to the fact that water molecules can act both as proton donors and proton acceptors (via the two lone-pairs). The most frequent coordinations are tetrahedral surroundings, i.e., types A, B, E, G, and H (class 2) (see Table 1). Trigonal and trigonal-pyramidal coordinations such as types C, D, and F (class 1) and I, J, and K (class T), respectively, are less frequent. Coordination numbers higher than 4, e.g., trigonal bipyramidal coordination. 

The first examples of heteroleptic divalent oiganotin compounds authenticated by X-ray crystal structure determinations are [2-pyridylbis(tri-methylsilyl)methyl]tin chloride 12 and [2-pyridylbis(trimethylsilyl)methyl] bis(trimethylsilyl)amidotin 13, for which the structures are schematically shown in Fig. 8 (73). The distorted trigonal pyramidal coordination geometry of the tin center in these compounds may be a result of the formation of a four-membered chelate ring. 

This structure, shown in Figure 1, is unusual in that it contains Y3+ in trigonal pyramid coordination, as well as copper in capped square-planar coordination (alternatively described as a square-based pyramid), the latter feature being shared with the structure of the superconducting phase. 

The reaction of 3b and 3c with (Me3Si)3CZnCl (5) yields a doubly metalated phosphane 8 [9] and arsane 9, respectively, according to Eq. 6. Selected spectroscopic data is listed in Table 1. These derivatives show a trigonal pyramidal coordination sphere at the pentel atom E with angle sums of 319.4° (E = P, 8 [9]) and 310.9° (E = As, 9) as shown for 9 in Fig. 3. 

A trimeric compound, [Me3SiCH2ZnSe(2,4,6-Bu 3C6H2)]3, 19, contains a twisted six-membered ring and trigonal pyramidal coordination of selenium (differing from the sulfur analog which is planar) [37],... 

Subsequently, other [Ln CH(SiMe3)2 3] (Ln = Y, Ce, Lu) have been synthesized by tiie same route, and there appears no reason why the others can be made, except possibly the Lu and Yb compounds. " These pentane-soluble compounds have trigonal pyramidal coordination geometries. One short Ln- CH3 contact is observed in the crystal structure of each molecule, ascribed to an agostic Si interaction (rather than yC- -H- Ln interaction). Reactions of some of these compounds with protic molecules... 

Fig. 26. Layers based on triangular (a) and trigonal-pyramid coordination (b-g). (a) BN-type layer (b) GeTe-type layer (c) AS2O3 layers in KAS4O6I and NH4AS2O3CI.5H2O (d) idealized layers of claudetite II, As203(li2) (e) idealized layer of orpiment, AS2S3 (f) strongly idealized version of the...

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