Physical Chemistry Third Edition

884 21 The Electronic Structure of Polyatomic Molecules

EXAMPLE21.8

Describe the bonding in the iodine trifluoride molecule. Apply the VSEPR rule that lone pairs

require more space than bonding pairs of electrons and are therefore placed in equatorial rather

than axial positions.

Solution

The valence shell of the iodine atom is the fifth shell, and the 4dsubshell is full, so thedorbitals

in the hybrid orbitals are from the same shell. We denote the five 5sp^3 dhybrids as follows:

The orbital region ofψ 5 sp (^3) d(1)points in the direction of the positivezaxis, that ofψ 5 sp (^3) d(2)
points in the direction of the negativezaxis, that ofψ 5 sp (^3) d(3)points in the direction of the
positivexaxis, that ofψ 5 sp (^3) d(4)points between the positiveyaxis and the negativexaxis with
an angle of 120◦from that ofψ 5 sp (^3) d(3), and that ofψ 5 sp (^3) d(5)points between the negativey
axis and the negativexaxis with an angle of 120◦from that of bothψ 5 sp (^3) d(3)andψ 5 sp (^3) d(4).
We place the fluorine atoms on the positive and negativezaxis and on the positivexaxis. In
each fluorine atom we form 2sp^3 hybrids and orient them so that one of the orbital regions
points toward the iodine atom. We form three sigma bonds with a 5sp^3 dhybrid on the iodine
atom and a 2sp^3 hybrid on a fluorine atom. The other orbitals on the fluorine atoms are occu-
pied by nonbonding electrons. All of the inner-shell orbitals on the iodine atom are occu-
pied by nonbonding electrons, as are the remaining hybrids on the iodine atom,ψ 5 sp (^3) d(4)
andψ 5 sp (^3) d(5). This description should agree fairly well with the energy-localized wave
function since it allows the lone pairs maximum separation from bonds and other lone
pairs.
Hybrid orbitals can be formed from onesorbital, twoporbitals, and onedorbital.
These orbitals point in the directions of thexandyaxes, and correspond to a square
planar electron geometry. If thedorbital is from the same shell as thesandporbitals,
these orbitals are calledsp^2 dorbitals, and if thedorbital is from the next lower shell,
they are calleddsp^2 orbitals. There are various platinum complexes that exhibit square
planar molecular geometry. Hybrid orbitals can also be formed from twodorbitals,
onesorbital, and threeporbitals. These orbitals are calledsp^3 d^2 if thedsubshell is
from the same shell as thesandporbitals ord^2 sp^3 if thedsubshell is from the shell
below that of thesandporbitals. These six orbitals have symmetry axes pointing
along the positive and negative Cartesian coordinate axes, the directions of the apices
of a regular octahedron, and the electron geometry or molecular geometry is called
octahedral. When these hybrids are used in a molecule wave function, all of the bond
angles are equal to 90◦. Examples of molecules with this “octahedral” electron geometry
are SF 6 , XeF 4 , and various complexes of transition metals.
PROBLEMS
Section 21.5: The Valence-Bond Description of
Polyatomic Molecules
21.19Using the modified valence-bond method, describe the
bonding of the BeCl 2 molecule. Predict the degree of
ionic character in the bonds. Look up the
electronegativity of Be if necessary.
21.20Using the valence-bond method describe the
bonding in H 2 CO (formaldehyde or methanal).
21.21cis-Diamminedichloroplatinum (II) (cisplatin) is a
chemotherapy drug that is commonly used against
testicular cancer. The molecule has a square planar
conformation with the two chlorines and the two