Trigonal pyramid configuration

Structure analysis of 61 [MgBrCthf) ] revealed a Pba trigonal-pyramidal configuration of the molecule with the electron pair occupying the apex position, resulting in the large pyramidalization at the central anionic Pb atom (279°). 

The correct answer is (C). BF3 has a trigonal planar configuration, and XeF4 has a square planar configuration. NH3 has a trigonal pyramidal configuration. 

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) ... 

Fig. 7. Structures of five-coordinate Cu2+ from first principles molecular dynamics. A Berry twist mechanism for the interconversion of the two structures is shown (from left to right) the reorientation of the main axis of a square pyramidal configuration by pseudo-rotations via a trigonal bipyramidal configuration. The grey atoms in the plane of the trigonal bipyramid are all candidates for becoming apical atoms in a square pyramid.

I also have a comment about the trigonal bipyramid vs. square pyramid configurations. Little physical motion of atoms is required to go from one to the other. There can be, I think, a tendency to take symmetry arguments too literally as black and white situations. The full symmetry of these complexes is considerably less than the symmetry one derives just from the arrangements of atoms next to the central ion. I do think it is possible to depend too much on exact symmetry designations. 

Nickel, on the other hand, on alumina and on silica supports was found to have only five nearby sulfurs (square pyramidal) with Ni-Mo coordination numbers from 1 to 1.5. Ni-Mo-S supported on carbon was observed to have Ni-S coordination numbers of 6 in a trigonal-prismatic configuration. In addition, Ni (at low Ni concentrations) was found to have one nearby Ni, which could indicate that, in some catalysts, Ni is present as pairs on the MoS2 surface. The overall structure of the Ni-Mo-S was believed to be similar to that of millerite (i.e., Ni is located in the center of the MoS edge in a square-pyramidal configuration, with one sulfur extending perpendicular to the surface) (62-64). 

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