Inorganic Chapter2

8/12/2019 Inorganic Chapter2

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Chapter 2 - Basic Concepts: moleculesChapter 2 - Basic Concepts: molecules

Bonding models:

Valence-Bond Theory (VB) and Molecular Orbital Theory (MO)

acceptor donor

Lewis acids and bases

When both of the electrons in the covalent bond formed by a Lewisacid-base reaction come from the same atom the bonds are calledcoordinate covalent bonds.

Drawing Lewis Structures1. Find the sum of valence electrons of all atoms in the polyatomic ion

or molecule. If it is an anion, add one electron for each negative charge.

If it is a cation, subtract one electron for each positive charge.

2. The central atom is the least electronegative element that isnt

hydrogen. Connect the outer atoms to it by single bonds.

3. Fill the octets of the outeratoms.

4. Fill the octet of the central atom.

5. If you run out of electrons before the central atom has an octet -form multiple bonds until it does.

Then assign formal charges. For each atom, count the electrons in lone pairs and half the electrons it shares with other atoms.

Subtract that from the number of valence electrons for that atom: The difference is its formal

charge.

The best Lewis structure

is the one with the fewest charges.

puts a negative charge on the most electronegative atom.


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Valence-Bond Theory (VB)

...332211cov +++= cccalent

H H H-H+

...

12

3

2

2

2

1 +++

=

cccN

)( 21cov +== + Nalent

)( 21 = N

Improvements can be made by:

allowing for the fact that each electron screens the other from thenuclei to some extent.

taking into account that both electrons 1 and 2 may be associatedwith either HA orHB, i.e. allowing for the transfer of one electron orthe other to form a pair of ions HA

+HB- or HA

-HB+

)]()[( 4321 +++=+ cN

)]([ cov ionicalentmolecule cN +=

HA(1)HB(2) HA(2)HB(1) [HA(1)(2)]-HB

+ HA+[HB(1)(2)]

-

H H H+ H- H- H+


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Valence-Bond Theory (VB) - Shortcomings

F F F+ F- F- F+

Consider Lewis structure of F2, N2, O2. VB theory assumed electronsare paired whenever possible.

Diamagnetism all electrons are spin-paired and results in a compoundbeing repelled by a magnetic field.

Paramagnetism a property of compounds with one or more unpairedelectrons that results in a compound being drawn into a magnetic field.

Molecular Orbital Theory (MO), homonuclear

Begin by placing nuclei in their equilibrium positions, then calculate the molecular orbitals

(regions of space spread over the entire molecule) that a single electron might occupy.

Each MO arises from interactions between atomic centers

interactions allowed if the symmetries of the atomic orbitals are compatible with one

another

efficient if the region of overlap between the atomic orbitals is significant

efficient if the atomic orbitals are relatively close in energy.

The number of MOs that can be formed must equal the number of atomic orbitals (AOs)

of the constituent atoms.

][ 21)( +== NMOphaseinMO

][ 21**

)( == NMOphaseofoutMO


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Schematic representations of the bondingand antibonding molecular orbitals in H2

The bond order is one half the difference betweenthe number of bonding and antibonding electrons.


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Overlap of p atomic orbitals


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Overlap of p atomic orbitals, cont.

MO Diagram second row X2 molecule.


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Diamagnetic1159141F2

Paramagnetic2498121O2

Diamagnetic3945110N2

Diamagnetic2607124C2

Paramagnetic1297159B2

-0--Be2

Diamagnetic1110267Li2

Magnetic

Properties

Bond

Order

Bond dissociation

enthalpy / kJ mol-1Bond distance

/pm

Diatomic


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Polar Covalent Bonds

electric dipole moment,

= qed


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Polarity

MO Theory: heteronuclear diatomic

)]()[( 21 yxMO ccN += )]()[(* 43 yxMO ccN +=


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Heteronuclear diatomic: HF

Heteronuclear diatomic: CO

LUMO

HOMO


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Molecular Shapes and VSEPR Model

In a molecule with formula EXn, each E-X bond isstereochemically significant.

Electron pairs, whether they be bonding ornonbonding, repel each other.

Electron-electron repulsions decrease in thesequence: lone pair-lone pair>lone pair-bondingpair > bonding pair-bonding pair

When there is a E-X multiple bond, electron-electron repulsions decrease in the sequence:

triple bond-single bond > double bond-single bond> single bond-single bond.

Repulsions depend on the electronegativities of Eand X; electron-electron repulsions are less themore the E-X bonding electron density is drawnaway from the central atom.


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ClF3

Stereoeisomerism

Isomers: Chemical species having the same atomic composition(same number and types of atoms) but different atomconnectivities or stererochemical arrangement.

Stereoisomers: Isomers thathave the same number and typesof chemical bonds but differing inthe spatial arrangement of thosebonds.

Structural isomers: Isomersthat have a differing numbersand types of chemical bonds.

1) Optical isomers (enantiomers):Contain different spatialarrangements that give chirality;possess nonsuperimposablemirror images.

2) Geometric isomers(diastereoisomers): Isomers withdiffering spatial arrangements thatresult in different geometries. Arenot mirror images of one another.

1) Coordination isomers: Isomersthat differ due to an interchangeof ligands among coordinationspheres.

2) Ionization isomers: Isomers

that differ by interchange ofgroups between coordinationspheres and countercations.

3) Linkage isomers:

Isomers that differ by the bondingsite used by an ambidentate ligand.


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Stereoisomerism

MA3B3

Inorganic Chapter2

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