Introduction_to_Inorganic_Chemistry_Coordination_Chemistry_and_Crystal_Field_Theory.pdf

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Introduction to Inorganic Chemistry/CoordinationChemistry and Crystal Field Theory1Chapter 5: Coordination Chemistry and Crystal Field Theory25.1 Counting electrons in transition metal complexes35.2 Crystal field theory45.3 Spectrochemical series55.4 π-bonding between metals and ligands65.5 Crystal field stabilization energy, pairing, and Hund's rule75.6 Non-octahedral complexes85.7 Jahn-Teller effect95.8 Tetrahedral complexes105.9 Stability of transition metal complexes115.10 Chelate and macrocyclic effects125.11 Ligand substitution reactions135.12 Discussion questions145.13 Problems155.14 ReferencesCoordination compounds(orcomplexes) are molecules and extended solids that contain bonds between atransition metalion and oneor moreligands. In forming thesecoordinate covalent bonds, the metal ions act as Lewis acids and the ligands act as Lewis bases.Typically, the ligand has a lone pair of electrons, and the bond is formed by overlap of the molecular orbital containing this electron pairwith the d-orbitals of the metal ion. Ligands that are commonly found in coordination complexes are neutral molecules (H2O, NH3,organic bases such as pyridine, CO, NO, H2, ethylene, and phosphines PR3) and anions (halides, CN-, SCN-, cyclopentadienide (C5H5-),H-, etc.). The resulting complexes can be cationic (e.g., [Cu(NH3)4]2+), neutral ([Pt(NH3)2Cl2]) or anionic ([Fe(CN)6]4-). As we will seebelow, ligands that have weak or negligible strength as Brønsted bases (for example, CO, CN-, H2O, and Cl-) can still be potent Lewisbases in forming transition metal complexes.With ligands that are Lewis bases, coordinate covalent bonds (also called dative bonds) are typically drawn as lines, or sometimes asarrows to indicate that the electron pair "belongs" to the ligand X:In counting electrons on the metal (described below), the convention is to assign both electrons in the dative bond to the ligand, althoughin reality the bonds are typically polar covalent and electrons are shared between the metal and the ligand.ContentsChapter 5: Coordination Chemistry and Crystal FieldTheory
Whenwritingouttheformulasofcoordination compounds, we use squarebrackets [...] around the metal ions and ligands that are directlybonded to each other. Thus the compound [Co(NH3)5Cl]Cl2contains octahedral [Co(NH3)5Cl]2+ions, in which fiveammonia molecules and one chloride ion are directly bonded tothe metal, and two Cl-anions that are not coordinated to themetal.History.Coordination compounds have been known forcenturies, but their structures were initially not understood. Forexample, Prussian Blue, which has an empirical formulaFe7(CN)18•xH2O, is an insoluble, deep blue solid that has beenused as a pigment since its accidental discovery by Diesbach in1704. Prussian Blue actually contains Fe3+cations and[Fe(CN)6]4-anions, and a more descriptive formulation is(Fe3+)4([Fe(CN)6]4-)3•xH2O. Simpler compounds such as theammonia complex of Co3+were known to chemists but did not fittheexpectedbehaviorofionicsolids.Forexample,

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Inorganic Chemistry, Coordination complex

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