By Vincenzo Balzani, Department of Chemistry “G. Ciamician”, University of Bologna, Italy

2a Electronic Structure 2a-1 Crystal Field Theory 2a-2 Ligand Field Theory 2a-3 Molecular Orbital Theory 2b Types of Excited States and Electronic Transitions 2c Absorption and Emission Bands 2c-1 Ligand-Field Bands 2c-2 Charge-Transfer Bands 2c-3 Intra-Ligand Bands 2c-4 A Remark 2c-5 Band width 2d Jablonski Diagram 2e Photochemical Reactivity 2e-1 Ligand-Field Excited States 2e-2 Charge-Transfer Excited States 2e-3 Intra-Ligand Excited States 2e-4 Remarks 2f Electrochemical Behavior 2f-1 Ligand Dissociation Processes 2f-2 Structural Changes 2f-3 Reversible Processes 2g Polynuclear Metal Complexes 2g-1 Supramolecular Species or Large Molecules? 2g-2 Electronic Interaction and Intervalence-Transfer Transitions

Transition metal complexes [9901, 0301], which are the most common coordination compounds, may be cationic, anionic or non-ionic species, depending on the charges carried by the central metal atom and the coordinated groups. These groups are usually called ligands. Ligands may be attached to the central atom through one or more atoms, i.e. they may be mono-or poly-dentate. The total number of attachments to the central atom is called coordination number. Coordination numbers of 2 to 10 are known, but most coordination compounds exhibit a coordination number of 6 or 4. A given metal ion does not necessarily have only one characteristic coordination number and, moreover, a given coordination number may give rise to several types of spatial arrangements. For example, Ni(II) forms both octahedral six-coordinated and tetrahedral or square-planar fourcoordinated complexes. Coordination compounds that contain ligands of two or more types are called mixed-ligand complexes.