Coordination geometry trigonal bipyramid

As a consequence of the anticipated coordination geometry ( -trigonal bipyramidal) of the tin center in 9, this atom becomes an additional chiral center. Since this compound is prepared starting from a racemic precursor, in principle three different diastereoisomers A, B, and C are expected (Fig. 6). 

The nitrosyls RuH(NO)(PR3)3 are 5-coordinate with trigonal bipyramidal structures and linear Ru-N-O geometries the hydride and nitrosyl ligands occupy the apical positions (for RuH(NO)(PPh3)3, z/(Ru-H) 1970 cm-1, i/(N—O) 1640 cm-1 H NMR, 8 = +6.6 ppm for the hydride resonance). The high-field NMR line is a quartet showing coupling with three equivalent phosphines, which would not be possible in a square pyramidal... 

Until the 1950s, this was thought to be quite rare. Now, however, five-coordinate complexes are known for practically all the d block elements, and for all d" configurations. Two idealised geometries are found in five-coordination the trigonal bipyramid (TBP) and the square pyramid (SP), shown below ... 

Figure 11.1. Schematic representation of typical silver (central atom) coordination environments with different coordination numbers and geometries (a) linear two-coordinate (b) triangular three-coordinate (c) T-shaped three-coordinate (d) square planar four-coordinate (e) tetrahedral four-coordinate (f) trigonal bipyramidal five-coordinate (g) tetragonal pyramidal five-coordinate (h) octahedral six-coordinate (i) trigonal prism six-coordinate (j) seven-coordinate (k) tetragonal prism eight-coordinate.

Five-coordinate species represent a rare class of coordination compounds. This rather unusual coordination number is a delicate balance between electronic factors which favor disproportion into four- and sbc-coordinate species and directional covalent bonds which help to stabilize them. The two limiting geometries (trigonal bipyramidal and square pyramidal) are therefore similar in energy and easily interconverted. As a result, five-coordinate species can exist as fluxional molecules, where their ligands can exchange places rapidly on the NMR time scale. 

Many of the nitrosyls studied are 5-coordinate, and analysis of crystallographic results indicates that, in general, in the trigonal bipyramid structures NO is found in the equatorial position in a linear geometry whereas in a square pyramidal structure, there is a bent M—N—O linkage in an apical position. A further point of interest is that in compounds like Ir(NO)Cl2(PPh3)2, the nitrosyl group bends in the more hindered (P—Ir—P) plane. 

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