Coordination Compounds: AP Material

Coordination Compounds Transition metals characteristically form coordination compounds These compounds are often coloured and often paramagnetic

Structure Coordination compounds are composed of: A complex ion (a transition metal with its attached ligands) Counterions (anions or cations needed to produce a compound with no net charge) [Co(NH3)5Cl]Cl2

A typical coordination compound [Co(NH3)4]Cl2 the brackets indicate the composition of the complex ion Co(NH3)42+ the two Cl- counterions are shown outside the brackets In this compound the four NH3 molecules act as ligands

Coordination Number and Oxidation State Coordination compounds have 2 types of valences (combining ability): Secondary valence – the ability of a metal to bind to Lewis bases (ligands) to form complex ions (coordination number) Primary valence – the ability of the metal ion to form ionic bonds with oppositely charged ions (oxidation state)

Coordination Number The number of bonds formed by metal ions to ligands in complex ions Varies from 2 to 8 depending on the size, charge and electron configuration of the transition metal ion 6 is the most common coordination number

Typical Coordination Numbers for Some Common Metal Ions Coord # s M2+ M3+ Coord #s Cu+ 2, 4 Mn2+ 4, 6 Sc3+ 6 Ag+ 2 Fe2+ Cr3+ Au+ Co2+ Co3+ Ni2+ Au3+ 4 Cu2+ Zn2+

Coordination Number and Geometry If the coordination number is 2, the geometry is linear

If the coordination number is 4, the geometry can be either tetrahedral or square planar

If the coordination number is 6, the geometry is octahedral

Ligands A ligand is a neutral molecule or ion having a lone pair of electrons that can be used to form a bond to a metal ion A ligand acts as a Lewis base The metal ion acts as a Lewis acid The resulting bond is often a coordinate covalent bond

Monodentate or Unidentate Ligand A mono- or unidentate ligand can form one bond to a metal ion (root: “one tooth”) Examples of monodentate ligands are: H2O CN- SCN- X-(halides) NH3 NO2- OH-

Chelating Ligands or Chelates Chelating ligands (from the Greek word chele for “claw”) have more than one atom with a lone pair of electrons and can thus form more than 1 bond with the same metal Bidentate ligand – a ligand that can form 2 bonds to a metal ion A very common bidentate ligand is ethylenediamine (abbreviated en)

Polydentate ligand – ligands that can form more than 2 bonds Most common example: ethylenediaminetetraacetate (EDTA) - can form 6 bonds thus is a hexadentate ligand

EDTA – a hexadentate ligand

EDTA - Uses Based on the large number of coordination sites (6), EDTA forms very stable complex ions with most metal ions It is used as a “scavenger” to remove toxic heavy metals from the human body It is also found in numerous consumer products: soda, beer, salad dressings, soap and most cleaners as it ties up trace metal ions that would otherwise catalyze decomposition and produce unwanted precipitates.

Naming Coordination Compounds As with any ionic compound, the cation is named before the anion. In naming a complex ion, the ligands are named before the metal ion. In naming ligands, an o is added to the root name of an anion. For example, the halides as ligands are called fluoro, chloro, bromo, and iodo; hydroxide is hydroxo, cyanide is cyano, etc. For a neutral ligand the name of the molecule is used, with the following exceptions:

Neutral Molecule Name of ligand H2O Aqua NH3 Ammine CH3NH2 Methylamine CO Carbonyl NO Nitrosyl

The prefixes mono-, di-, tri-, tetra-, penta- and hexa- are used to denote the number of simple ligands. The prefixes bis-, tris-, tetrakis-, and so on are also used for more complicated ligands or for ones that already contain di-, tri-, etc. The oxidation state of the central metal atom is designated by a Roman numeral in parenthesis.

When more than one type of ligand is present, they are named alphabetically. Prefixes do not affect order. If the complex ion has a negative charge, the suffix –ate is added to the name of the metal. Sometimes the Latin name is used to identify the metal.

Metal Name in an Anionic Complex Iron Ferrate Copper Cuprate Lead Plumbate Silver Argentate Gold Aurate Tin Stannate

Name the following coordination compounds a) [Co(NH3)4(NO2)Cl]Cl b) [Pt(NH3)3Br]Cl c) [Co(NH3)5Cl]Cl2 d) K3[Fe(CN)6] e) [Fe(en)2(NO2)2]2SO4

Answers Tetraaminechloronitrocobalt(III) chloride Triamminebromoplatinum(II) chloride Pentaamminechlorocobalt(III) chloride Potassium hexacyanoferrate (III) Bis(ethylenediamine)dinitroiron(III) sulfate