Trigonal bipyramidal shape

The electron pairs are arranged in a trigonal bipyramidal shape. 

Formula TaCls MW 358.21 shows a dinuclear structure, Ta2Clio, but in vapor phase it exists as a mononuclear, TaCls, having a trigonal bipyramidal shape. 

In PF5, five sp d orbitals are needed to provide a trigonal-bipyramidal shaped molecule. The P (phosphorus) atom has a single electron in each of the five sp d orbitals ready to overlap a 2p orbital in each of the five F (fluorine) atoms to form the molecule. 

The five-coordination compounds show some more exotic possibilities. PF5 has a trigonal bipyramidal shape with inequivalent axial and equatorial positions. The lone pair in SF4 chooses an equatorial position since it can do less damage there— it make a 90° angle with two F atoms, whereas an axial position would make 90° angles with three F atoms. This leaves SF4 with a shape resembling a distorted. seesaw. The two lone pairs in CIF3 are most stable when located in two equatorial positions, separated by 120". This leaves Cll 3 in a distorted tee shape. The eomplex that forms between 1 and l> in iK iieous solution is a linear ion. 

This last compound, treated as a mixed metal-carbon cluster Ic4(( ()) iT R (R = the one-electron carboxylate ligand COOMe) contains six skeletal bond pairs to hold together its five skeietai atoms (one carbon atom and four metal atoms). It is therefore a closo cluster, with the expected trigonal bipyramidal shape defined by its skeietai carbon and metai atoms. 

CIF3 This is an XY3 interhalogen. The central Cl atom is surrounded by five electron pairs two are lone electron pairs and three are boding pairs resulting in a trigonal bipyramidal shape and a distorted T-shaped molecular geometry. 

Presumably, this effect makes it difficult to assert that a single shape is universally explanatory. If ammonia engages in a reaction under conditions where it has a trigonal bipyramidal shape, than we probably could not say it reacts in a certain way because it has the shape of a trigonal pyramid. Chemists routinely appeal to shape to explain how a molecule reacts.Some shape might be explanatory in every explanation, but the shape will be different in different explanations. 

With all five positions occupied by bonded atoms, the molecule has the trigonal bipyramidal shape (AXj), as in phosphorus pentachloride (PCI5) ... 

Step 4. Molecular shape Five electron groups and no lone pairs give the trigonal bipyramidal shape (AX5) ... 

For the trigonal bipyramidal shape of the PCI5 molecule, for example, the VB model proposes that the one 35, the three 3p, and one of the five 3d orbitals of the central P atom mix and form five sp d hybrid orbitals, which point to the vertices of a trigonal bipyramid (Figure 11.6). Seesaw, T-shaped, and linear molecules have this electron-group arrangement with lone pairs in one, two, or three of the central atom s sp d orbitals, respectively. 

Figure 16.15 Nature of the transition state in the reaction between CHsBr and OH . Note the partial (elongated) C—O and C—Br bonds and the trigonal bipyramidal shape of the transition state of this reaction.

The trigonal bipyramidal shape merits a special note. Contrary to the other shapes, it possesses two types of bonds the two axial bonds located at 180° from each other, and the three equatorial bonds located perpendicularly to the axis and at 120° from each other. 

All the Group 5A elements except nitrogen can form molecules with five covalent bonds (of general formula MX5). Nitrogen cannot form such molecules because of its small size. The MX5 molecules have a trigonal bipyramidal shape (see Fig. 20.9) as predicted by the VSEPR model, and the central atom can be described as dsp hybridized. 

The NbBrs molecule has five pairs of bonding electrons. The trigonal bipyramidal shape minimizes the repulsion of these shared electron pairs. 

Carbonium ions have five bonds to the carbon atom and a complete outer shell of eight electrons. A simple example is the ion CH5 which has a trigonal bipyramidal shape. Ions of this type are transient species. They can be product by electron impact and detected by mass spectroscopy. 

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