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Metal Complexes -- Chapter 25 (Sections 25.3 - 25.5) 1. Metal Complex -- consists of a set of ligands that are bonded to a central metal coordinate covalent bonds. e.g., Cu2+ + 4 L → ion CuL42+ (L = NH3, H2O, etc.) (a) Ligands are Lewis Bases and can be: • monodentate -- one donor atom e.g., H2O, NH3, Cl-, OH-, etc. • bidentate -- two donor atoms e.g., ethylenediamine, NH2-CH2-CH2-NH2 "en" H2C H2C H2N: : NH2 = H2N: : NH2 M M • polydentate -- more than two donor atoms e.g., EDTA -- ethylenediaminetetraacetic acid (6 donor atoms) O .. :O .. C CH2 CH2 :N .. :O .. C CH2 O O CH2 .. : H2C C O .. N: .. H2C C O : .. O 1 = EDTA4- by (b) Writing Formulas of complex ions metal ion first, then ligands total charge = sum of metal ion + ligands e.g., metal ion ligand complex Cu2+ H2O Cu(H2O)42+ Co3+ NH3 Co(NH3)63+ Fe3+ CN- Fe(CN)63- (c) Chelate Effect - complexes with bi- or polydentate ligands are more stable than those with similar monodentate ligands e.g., Ni(en)33+ is more stable than Ni(NH3)63+ 3+ N N N N N Ni N (d) Nomenclature -- study rules and examples in book! Complex Ni(CN)42- Name tetracyanonickelate(II) ion CoCl63- hexachlorocobaltate(III) ion Co(NH3)4Cl2+ Na3[Co(NO2)6] tetraamminedichlorocobalt(III) ion [Cr(en)2Cl2]2SO4 dichlorobis(ethylenediamine)chromium(III) sulfate sodium hexanitrocobaltate(III) 2 2. Coordination Number and Structure Coord # = number of donor atoms attached to the metal center (a) Two-Coordinate Complexes -- linear structures rare except for Ag+ e.g., Ag(NH3)2+ and Ag(CN)2(b) Four-Coordinate Complexes -- two structural types • Tetrahedral structures -- common for ions with filled d subshells, e.g., Zn2+ as in Zn(OH)42- 2- OH Zn OH HO OH • Square Planar structures -- common for d8 metal ions (Ni2+, Pd2+, Pt2+) and for Cu2+ e.g., Cl Cl 2- H3N Pt Cl NH3 2+ Cu H3N Cl NH3 (c) Six-Coordinate Complexes -- the most common! "always" octahedral structures, e.g., 3+ N N N Ni Cl N N H3N H3N N 3 Cr Cl NH3 Cl 3. Isomers of Coordination Complexes Isomers -- same chemical composition (formula) but different structures (due to either the arrangement of atoms or 3-D shape) (a) Linkage isomers same ligand, with different donor atoms e.g., In [Co(NH3)5NO2]2+, the NO2- ligand can bind to Co through N ("nitro") or O ("nitrito"). H3N NH 3 O H3N Co N O H3N NH 3 2+ H 3N H 3N NH3 Co H 3N nitro complex 2+ O N O NH3 nitrito complex 4 (b) Geometrical isomers • Square Planar complexes Cl Pt Cl NH3 Cl NH3 H3N NH3 Pt cis Cl trans "cisplatin" (anticancer drug) (no biological activity) • Octahedral complexes Cl H3N H3N Cr Cl NH3 Cl H3N H3N NH3 cis NH3 NH3 Cr Cl trans (c) Enantiomers -- non-superimposable mirror images also called "optical isomers" N N N Co N N Cl N Cl Cl Co Cl 5 N N 4. Crystal Field Theory (Bonding in Transition Metal Complexes) Metal complexes are usually highly colored and are often paramagnetic – such facts can be explained by a "d-orbital splitting diagram" dz2 dx2-y2 eg ∆ = crystal field energy splitting energy dxy dxz dyz t2g d orbitals of the metal ion in an octahedral field of ligands d orbitals of the free metal ion The size of ∆ depends on • the nature of the ligand "spectrochemical series" -- ∆ decreases: CN- > NO2- > en > NH3 > H2O > OH- > F- > Cl- > Br"strong field ligands" "weak field ligands" • the oxidation state of the metal ∆ is greater for M3+ than for M2+ • the row of the metal in the periodic table for a given ligand and oxidation state of the metal, ∆ increases going down in a group e.g., ∆ is greater in Ru(NH3)63+ than in Fe(NH3)63+ Colors of metal complexes are due to electronic transitions between the t2g and eg energy levels 6 d orbital splitting diagrams for octahedral complexes dz2 dz2 dx2-y2 dx2-y2 eg large ∆ "low spin" eg small ∆ "high spin" dxy dxz dyz t2g dxy Fe(H2O)62+ dxz dyz t2g Fe(CN)64- CN- is a stronger field ligand than is H2O which leads to a greater ∆ value (i.e., a greater d orbital splitting) as a result, Fe(H2O)62+ is a "high spin" complex and is paramagnetic (4 unpaired electrons) while, Fe(CN)64- is a "low spin" complex and is diamagnetic (no unpaired electrons) the CN- complex with the larger ∆ value absorbs light of higher energy (i.e., higher frequency but shorter wavelength) OMIT: d orbital splitting diagrams for other geometries (i.e., tetrahedral and square planar) 7