Trigonal bipyramidal molecular shape

Predict the electron-domain and molecular geometries of (a) Brp3, (b) ICI4. Answws (a) trigonal bipyramidal, T-shaped (b) octahedral, square planar... 

C09-0129. Tellurium compounds, which are toxic and have a hideous stench, must be handled with extreme care. Predict the formula of the tellurium-fluorine molecule or ion that has the following molecular geometry (a) bent (b) T-shaped (c) square pyramid (d) trigonal bipyramid (e) octahedron and (Q seesaw. 

The Lewis formula predicts 5 electron groups around the central Br atom and a trigonal bipyramidal electronic geometry. There are two lone pairs on the Br atom, so the molecular geometry is T-shaped (Section 8-11). 

A theory which shows greater applicability to bonding in cluster compounds is the Polyhedral Skeletal Electron Pair Theory (PSEPT) which allows the probable structure to be deduced from the total number of skeletal bond pairs (400). Molecular orbital calculations show that a closed polyhedron with n vertex atoms is held together by a total of (n + 1) skeletal bond pairs. A nido polyhedron, with one vertex vacant, is held together by (n + 2) skeletal bond pairs, and an arachno polyhedron, with two vacant vertices, by (n + 3) skeletal bond pairs. Further, more open structures are obtainable by adding additional pairs of electrons. This discussion of these polyhedral shapes is normally confined to metal atoms, but it is possible to consider an alkyne, RC=CR, either as an external ligand or as a source of two skeletal CR units. So that, for example, the cluster skeleton in the complex Co4(CO)10(RCCR), shown in Fig. 16, may be considered as a nido trigonal bipyramid (a butterfly cluster) with a coordinated alkyne or as a closo octahedron with two carbon atoms in the core. 

S2.3 The Lewis structures and molecular shapes for XcFi and ICb are shown below. The XeFi Lewis structure has an octet for the 4 F atoms and an expanded valence shell of 10 electrons for the Xe atom, with the 8 + (2 x 7) = 22 valence electrons provided by the three atoms. The five electron pairs around the central Xe atom will anange themselves at the comers of a trigonal bipyramid (as in PF5). The three lone pairs will be in the equatorial plane, to minimize lone pair-lone pair repulsions. The resulting shape of the molecule, shown at the right, is linear (i.e., the F-Xe-F bond angle is 180°). 

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. 

Molecular Shapes with Five Electron Groups (Trigonal Bipyramidal Arrangement)... 

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

The VSEPR model we discussed in Chapter 10 accounts for molecular shapes by assuming that electron groups minimize their repulsions, and thus occupy as much space as possible around a central atom. But it does not explain how the shapes arise from interactions of atomic orbitals. After all, the orbitals we examined in Chapter 7 aren t oriented toward the comers of a tetrahedron or a trigonal bipyramid, to mention just two of the common molecular shapes. Moreover, knowing the shape doesn t help us explain the magnetic and spectral properties of molecules only an understanding of their orbitals and energy levels can do that. 

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