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Unformatted text preview: What Is A Coordination Compound? A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions (called ligands ) bond to a central metal atom (or ion) by coordinate covalent bonds . Ligands are Lewis bases - they contain at least one pair of electrons to donate to a metal atom/ion. Ligands are also called complexing agents . Metal atoms/ions are Lewis acids - they can accept pairs of electrons from Lewis bases. Within a ligand, the atom that is directly bonded to the metal atom/ion is called the donor atom . A coordinate covalent bond is a covalent bond in which one atom (i.e., the donor atom) supplies both electrons. This type of bonding is different from a normal covalent bond in which each atom supplies one electron. If the coordination complex carries a net charge, the complex is called a complex ion . Compounds that contain a coordination complex are called coordination compounds . Coordination compounds and complexes are distinct chemical species - their properties and behavior are different from the metal atom/ion and ligands from which they are composed. The coordination sphere of a coordination compound or complex consists of the central metal atom/ion plus its attached ligands. The coordination sphere is usually enclosed in brackets when written in a formula. The coordination number is the number of donor atoms bonded to the central metal atom/ion. Some Coordination Complexes example molecular formula Lewis base/ligand Lewis acid donor atom coordination number [Ag(NH 3 ) 2 ] + NH 3 Ag + N 2 [Zn(CN) 4 ] 2- CN- Zn 2+ C 4 [Ni(CN) 4 ] 2- CN- Ni 2+ C 4 [PtCl 6 ] 2- Cl- Pt 4+ Cl 6 [Ni(NH 3 ) 6 ] 2+ NH 3 Ni 2+ N 6 Common Structures Why do we want to know the structure for a coordination compound? The compound's structure (i.e., how the ligands are arranged around the metal atom) determines its physical and chemical properties. For example, a tetrahedral compound will behave differently than a square planar compound that contains the same metal atom and the same ligands. Consider the complex ion, [CoCl 4 ] 2- . The coordination number of [CoCl 4 ] 2- is equal to 4; thus, the structure of [CoCl 4 ] 2- might be: 1. tetrahedral- ligands attached at the corners of a tetrahedron 2. square planar- ligands attached at the corners of a square 3. something else? Coordination compounds with a coordination number (CN) of 4 are generally either tetrahedral or square planar . The [CoCl 4 ] 2- complex ion is experimentally known to have a tetrahedral structure. A Comparison of Tetrahedral and Square Planar Structures example sketch model [CoCl 4 ] 2- tetrahedral CN = 4 ligands attached at the corners of a tetrahedron [Ni(CN) 4 ] 2- square planar CN = 4 ligands attached at the corners of a square Coordination numbers range from 1 to 12, with 2, 4 and 6 being the most common. Linear and octahedral are the most common structures for coordination numbers 2 and 6, respectively....
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