Covalent Bonding Pt 3: Hybridization

Introduction We now know that atoms can bond covalently through the sharing of electrons VSEPR theory helps us predict molecular shapes. But, it does not explain what bonds are, how they form, or why they exist. Here, chemical bonding will be explained in terms of orbitals

Covalent Bonding Is Due to Orbital Overlap In a covalent bond, electron density is concentrated between the nuclei. Thus, we can imagine the valence orbitals of the atoms overlapping The region of orbital overlap represents the covalent bond

Overlapping Valence Orbitals Recall s and p orbitals S S orbitals are spherical. L = 0, mL = 0 Max of 2 electrons P orbitals consist of two lobes of electron density. L= 1, mL = -1, 0, 1 (3 suborbitals) Max of 6 electrons px py pz

Forming Sigma (σ) Bonds Energy 1s1 1s1 σ stabilization (energy drop) Two overlapping atomic orbitals form a molecular bonding orbital. A sigma (σ) bonding orbital forms when s-orbitals overlap.

Introduction to Hybridization Imagine the molecule CH4. We know that carbon has 4 valence electrons (2s22p2). However, when we fill our orbitals in order as according to Hund’s rule, we notice there are only enough unpaired electrons to make two bonds. Stay mindful of the fact that a covalent bond involves the sharing of unpaired electrons C ENERGY X 2p2 4 H 2s2 1s1 1s1 1s1 1s1

sp3 Hybridization So how does CH4 form? How can carbon make 4 bonds? To make four bonds, carbon hybridizes four of its atomic orbitals. This creates four equivalent sp3 hybrid orbitals, each containing one unpaired electron. The name “sp3” originates from the fact that the hybrid orbitals form as a result of the mixture of 1 s-orbital and 3 p-orbitals. Thus, each sp3 orbital is 25% s character and 75% p character

Illustration of sp3 Hybrid Orbitals and Orbital Overlap Orbital Mixing The four hybrid orbitals arrange themselves tetrahedrally around the nucleus.

Formation of Sigma Bonding Orbitals sp3 hybrid orbitals ENERGY C 1s1 1s1 1s1 1s1 atomic s-orbitals 4H σ bonding orbitals

Hybridization of Lone Electron Pairs Ex. What is the hybridization of Oxygen in H2O? The valence electron configuration of O is [He]2s2 2p4 O ENERGY 2p4 2 H 2s2 1s1 1s1 As you see, there are two unpaired O electrons. Does this mean that these two p-suborbitals can overlap with the two Hydrogen 1s orbitals without hybridizing??

•• •• X Hybridization of Lone Electron Pairs O 2 H O H H ENERGY 2p4 2p electrons 2s electrons O H H ENERGY 2p4 X 2 H BAD!! 2s2 1s1 1s1 No!! The reason is that we now have two sets of lone pairs of electrons that are substantially different in energy (2s and 2p). The orbitals will hybridize to form degenerate (equal energy) lone pairs.

Hybridization of Lone Electron Pairs The orbitals will hybridize to form degenerate (equal energy) lone pairs.

A Simple Trick To Remember Total Electron Domains Around Atom (Bond + LP) Hybridization 2 sp 3 sp2 4 sp3

Double and Triple Bonding How can orbital overlap be used to explain double and triple bonds? What kind of interactions are these? Lets look at formaldehyde, H2CO C O H •• sp2 The hybridization of carbon and oxygen is sp2 because each is surrounded by three electron domains. sp2

Forming Double Bonds We can see that the carbon atom needs to make three sigma bonds, so this will require three unpaired electrons to be spread across the three sp2 hybrid orbitals. This will leave one unhybridized p orbital with an electron in it.

Forming Double Bonds The oxygen is also sp2. It needs to form two lone pair, and also needs to form one sigma bond to carbon. This will require 5 electrons to be spread across the three sp2 hybrid orbitals, which will leave one unhybridized p orbital with an electron in it.

Forming Double Bonds The H s-orbitals overlap with C sp2 orbitals to create C-H sigma bonds. The remaining C sp2 orbital overlaps with the half filled O sp2 orbital to create the C-O sigma bond. The unhybridized p orbitals of C and O interact to produce a pi bond, which is slightly weaker than a sigma. DOUBLE BOND = 𝟏𝝈+𝟏𝝅

Triple Bonds :1 σ-bond and 2 π-bonds. Ex. C2H2 sp sp H C C H The carbons have sp hybridization. Each carbon requires two electrons to make two sigma bonds. This leaves two unhybridized p orbitals, each containing one electron.

Examples CH3CH2CHCHCH3 CH3CCCHCH2 How many σ and π bonds are in each of the following molecules? Give the hybridization of each carbon. CH3CH2CHCHCH3 CH3CCCHCH2

sp3d and sp3d2 hybridization Atoms like S, Se, I, Xe … etc. can exceed an octet because of sp3d and sp3d2 hybridization (combination of ns, np, and nd orbitals where n>3). This results in either trigonal bipyramidal or octahedral skeletal geometry sp3d sp3d2

Exceeding an Octet. Example: SF6 S needs to make six sigma bonds. Therefore, its 6 valence electrons must be spread across six hybrid orbitals. This means that the 3s orbital, all three of the 3p orbitals, and two of the 3d orbitals must mix to yield six sp3d2 hybrid orbitals. Note: The fluorines are sp3 hybridized.

Look Familiar ???

Examples: What is the hybridization of the central atom? CO2 H2CO CH3CCH IF5 PCl5 SeOF4