MOLECULAR ORBITALS - Chapter 5 - Expectations for MOs - Coordination Complexes

Oct 25, 2026

Chapter 5 expectations for molecular orbitals focus on drawing MO diagrams for homonuclear diatomic molecules, defining bond order, HOMO, LUMO, and the paramagnetic/diamagnetic distinction, and using symmetry to derive SALCs and match them to central atom orbitals. For molecules with three or more atoms, the material emphasizes predicting MO diagrams from interactions between central orbitals and SALCs, identifying bonding, antibonding, and nonbonding participation without requiring exact energy ordering or memorizing orbital energy levels. It then transitions to coordination complexes and crystal-field/ligand-field ideas: ligands donate electrons to a metal center as Lewis bases and acids, with bonding described via coordinate covalent/dative concepts and standard coordination-complex terminology, including coordination number, monodentate vs polydentate chelating ligands, chelate rings, and cis/trans stereoisomerism.

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Expectations for MOs

Coordination Complexes

Coordination Complex Terminology

Bonding and Lewis Acid/Base Donor-Acceptor Chemistry

Isomerism in Coordination Complexes

MOLECULAR ORBITALS Chapter 5 Expectations for MOs • Draw correct MO diagrams for homonuclear diatomic molecules • Know the definitions of bond order, HOMO, LUMO, paramagnetic, diamagnetic • Derive SALCs for a given molecule, be able to match SALCs to central atom orbitals based on symmetry • Predict an MO diagram for molecules with 3 atoms or more, based on interactions between central atom orbitals and SALCs. • We do not expect you to get the relative energy levels exactly right, however, you should properly indicate which orbitals and SALCS participate in bonding/ antibonding interactions and which orbitals and SALCs are non bonding • Orbital energy levels will be provided on the exam, you do not have to memorize these. • You do not need to know how to use the‘projection operator’ (in MFT) COORDINATION COMPLEXES MFT Chapters 6,9,10 Coordination complexes • Compounds that are composed of a metal atom or ion and one or more ligands (atoms, ions, or molecules) that donate electrons to the metal H 3N NH 3 Pt ~~ ~~ Cl Cl see MFT Ch 6 for more details Acid-Base Donor-Acceptor Chemistry • In a coordination compound, the metal center ( Mn+) acts as a Lewis acid and the ligand (L) acts as a Lewis base Mn+ L H 3N NH 3 Pt ~~ ~~ Cl Cl Pt 2+ Cl Cl N H H H H N H H Lewis acid Lewis base See Section 6.4 for more discussion Bonding in coordination complexes • You are familiar with covalent and ionic bonds • Covalent bond: electron pair in bond is shared between two atoms, typically one electron is donated from each atom • Ionic bond: electrostatic interaction between oppositely charge ions (electrons NOT shared) • In transition metal complexes, the bonds are described as coordinate covalent or dative bonds • In the Lewis acid-Lewis base interactions in these compounds, the electron pair in the ligand (the ligand HOMO – highest occupied molecular orbital) is interacting with an empty orbital on the metal center (the LUMO – typically d orbitals) • In this bond, the electrons are shared by the metal and ligand, but both electrons come from the ligand Coordination Complex Terminology: • The following slides are the main terminology/nomenclature things I expect you to know, you do not need to memorize all the nomenclature rules in Ch 9 • Coordination number: how many ligands are attached to the metal center H 3N NH 3 Pt ~~ ~~ Cl Cl B B A M A A B 4 5 6 Ligand terminology • Monodentate: ligand has only one point of attachment (‘one tooth’) H 3N ~~ ~~ NH 3 Pt ~~ ~~ Cl Cl All ligands in cis platin are monodentate Ligand terminology • Polydentate or chelating ligands: ligand contains multiple points of attachment to metal ions Cl Co • Chelate ring: how many atoms comprise a ligand-metal-ligand interaction in a chelating ligand NH 2 H 2 N NH 2 Cl N + Tetradentate ligand/chelate Bidentate ligand/chelate ‘chelate’comes from the Greek word khele for crab claw 5 Isomerism in coordination complexes • A certain ligand set may be able to arrange itself indifferent ways around a metal center • For full set of isomer types, see MFT Section 9.3 • Stereoisomers: cis and trans

FAQs

What should you be able to do for homonuclear diatomic molecules in MO theory?

Draw correct MO diagrams and use concepts like bond order, HOMO/LUMO, and paramagnetic versus diamagnetic behavior. The focus is on proper orbital participation in bonding, antibonding, and nonbonding interactions.

How are bonding interactions described in coordination complexes using Lewis acid/base chemistry?

The metal center acts as a Lewis acid (Mn+) and the ligand acts as a Lewis base (L). The ligand HOMO (electron pair) interacts with an empty metal orbital, typically metal d orbitals for transition metals.

What is the difference between monodentate and polydentate (chelating) ligands?

Monodentate ligands attach through one point to the metal, while polydentate/chelating ligands have multiple attachment points. Chelate ring size describes how many atoms form the ligand-metal-ligand interaction.

Oct 25, 2026

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