Hybrid Orbitals

Molecular Geometry and Bonding Theories

Hybrid Orbitals

Hybrid Orbitals

For polyatomic molecules we would like to be able to explain:

The number of bonds formed
Their geometries

sp Hybrid Orbitals

Consider the Lewis structure of gaseous molecules of BeF₂:

The VSEPR model predicts this structure will be linear
What would valence bond theory predict about the structure?

The fluorine atom electron configuration:

1s²2s²2p⁵

There is an unpaired electron in a 2p orbital
This unpaired 2p electron can be paired with an unpaired electron in the Be atom to form a covalent bond

The Be atom electron configuration:

1s²2s²

In the ground state, there are no unpaired electrons (the Be atom is incapable of forming a covalent bond with a fluorine atom
However, the Be atom could obtain an unpaired electron by promoting an electron from the 2s orbital to the 2p orbital:

This would actually result in two unpaired electrons, one in a 2s orbital and another in a 2p orbital

The Be atom can now form two covalent bonds with fluorine atoms
We would not expect these bonds to be identical (one is with a 2s electron orbital, the other is with a 2p electron orbital)

However, the structure of BeF₂ is linear and the bond lengths are identical

We can combine wavefunctions for the 2s and 2p electrons to produce a "hybrid" orbital for both electrons
This hybrid orbital is an "sp" hybrid orbital

The orbital diagram for this hybridization would be represented as:

Note:

The Be 2sp orbitals are identical and oriented 180° from one another (i.e. bond lengths will be identical and the molecule linear)
The promotion of a Be 2s electron to a 2p orbital to allow sp hybrid orbital formation requires energy.

The elongated sp hybrid orbitals have one large lobe which can overlap (bond) with another atom more effectively
This produces a stronger bond (higher bond energy) which offsets the energy required to promote the 2s electron

sp² and sp³ Hybrid Orbitals

Whenever orbitals are mixed (hybridized):

The number of hybrid orbitals produced is equal to the sum of the orbitals being hybridized
Each hybrid orbital is identical except that they are oriented in different directions

BF₃

Boron electron configuration:

The three sp² hybrid orbitals have a trigonal planar arrangement to minimize electron repulsion

NOTE: sp² refers to a hybrid orbital being constructed from one s orbital and two p orbitals. Although it looks like an electron configuration notation, the superscript '2' DOES NOT refer to the number of electrons in an orbital.

An s orbital can also mix with all 3 p orbitals in the same subshell

CH₄

Thus, using valence bond theory, we would describe the bonds in methane as follows: each of the carbon sp³ hybrid orbitals can overlap with the 1s orbitals of a hydrogen atom to form a bonding pair of electrons

NOTE: sp³ refers to a hybrid orbital being constructed from one s orbital and three p orbitals. Although it looks like an electron configuration notation, the superscript '3' DOES NOT refer to the number of electrons in an orbital.

ANOTHER NOTE: the two steps often observed when constructing hybrid orbitals is to 1) promote a valence electron from the ground state configuration to a higher energy orbital, and then 2) hybridize the appropriate valence electron orbitals to achieve the desired valence electron geometry (i.e. the correct number of hybrid orbitals for the appropriate valence electron geometry)

H₂O

Oxygen

Hybridization Involving d Orbitals

Atoms in the third period and higher can utilize d orbitals to form hybrid orbitals

PF₅

Similarly hybridizing one s, three p and two d orbitals yields six identical hybrid sp³d² orbitals. These would be oriented in an octahedral geometry.

Hybrid orbitals allows us to use valence bond theory to describe covalent bonds (sharing of electrons in overlapping orbitals of two atoms)
When we know the molecular geometry, we can use the concept of hybridization to describe the electronic orbitals used by the central atom in bonding

Steps in predicting the hybrid orbitals used by an atom in bonding:

1. Draw the Lewis structure

2. Determine the electron pair geometry using the VSEPR model

3. Specify the hybrid orbitals needed to accommodate the electron pairs in the geometric arrangement

NH₃

1. Lewis structure

2. VSEPR indicates tetrahedral geometry with one non-bonding pair of electrons (structure itself will be trigonal pyramidal)

3. Tetrahedral arrangement indicates four equivalent electron orbitals

Valence Electron Pair Geometry	Number of Orbitals	Hybrid Orbitals
Linear	2	sp
Trigonal Planar	3	sp²
Tetrahedral	4	sp³
Trigonal Bipyramidal	5	sp³d
Octahedral	6	sp³d²

1996 Michael Blaber