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lecture+15 - Coordination Compounds A coordination compound...

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Coordination Compounds A coordination compound consists of a complex ion + a counter ion. The complex ion consists of a central metal cation to which ligands are tightly attached. It is designated by brackets [ ] in writing the formula. The counter ion is not a ligand, and is not tightly attached. On dissolving in water, the counter ion dissociates freely, but the complex ion maintains its overall structure, with the ligands still attached to the central metal cation. Counter ions are needed to make a zero charge overall. If the complex ion is uncharged, then there is no counter ion. Complex ion: Transition metal ion + ligands Counter ion: may be positive or negative. If the complex ion is positive, the counter ion is negative. If the complex ion is negative, the counter ion is positive. Ligands are Lewis bases. Common ligands: CN - , H 2 O, Cl - , NH 3 Bidentate ligand: ethylenediamine (en), oxalato Polydentate ligand: ethylenediaminetetraacetato (EDTA) Nomenclature The cation is named before the anion The ligands are named before the central metal ion Ligand names: CN - cyano H 2 O aqua Cl - chloro NH 3 ammine Prefixes: di, tri, tetra, penta, hexa (number of simple ligands) bis, tris, tetrakis (number of complex ligands) Oxidation state of central metal ion (roman numeral) If more than one ligand, use alphabetical order If complex ion is negatively charged, use “ate” suffix for the central metal ion Consider the compound [Co(NH 3 ) 6 ]Cl 3
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What is the charge on [ ] (complex ion)? What is the oxidation number of cobalt? If excess AgNO 3 were added to one mole of this compound, how many moles of AgCl would precipitate? Name the compound. [Co(NH 3 ) 6 ]Cl 3 has 3 Cl - ions outside the complex ion. Thus the complex ion [ ] has a +3 charge, since the overall compound must be neutral. The complex ion [Co(NH 3 ) 6 ] had a +3 charge. The ligands, NH 3 , are neutral molecules. Thus the central Co ion has a +3 charge, making the overall charge on the complex ion +3. The oxidation number of Co is +3. [Co(NH 3 ) 6 ]Cl 3 , when in water consists of [Co(NH 3 ) 6 ] 3+ and 3Cl - . The 3Cl - outside the complex ion are called “counter ions” and are not ligands, and move freely throughout the solution. They can combine with Ag + to form AgCl precipitate. Thus excess AgNO 3 precipitates 3 moles of AgCl (from the 3 chloride counter ions). [Co(NH 3 ) 6 ]Cl 3 is named: Hexaamminecobalt(III) chloride Consider the compound [Co(NH 3 ) 5 Cl]Cl 2
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What is the charge on [ ] (complex ion)?
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