The importance of transition metal complexes in biochemistry is an extremely active area of research. Catalytic cofactors: Many reactions require trace elements as catalytic cofactors. For example: Zinc is required in over 200 reactions including synthesis of proteins, taste perception, prostrate reproductive health, metabolizing alcohol, and protecting against copper and heavy metal toxicity such as cadmium and lead. Zinc occurs in greater amounts than any other trace mineral except iron. Manganese is required to synthesize connective tissue and bones (collagen). Oxidation/reduction: Some metal ions, particularly iron, copper, and manganese are involved in the energy metabolism of cells. Iron is involved in the electron transport that ultimately converts oxygen to water. Copper participates in electron transport as well as synthesis of nerve membranes and formation of collagen. Oxygen binding and transport: Oxygen is carried by the red cells of the blood from the lungs bound to hemoglobin which contains iron at its active heme center. Oxygen is released to the tissues where it is picked up by a similar protein, myoglobin, before it accepts electrons and protons to form water. Metabolic regulation: Iron, copper and zinc can regulate the activities of protein and nucleic acid synthesis. Proper immune response requires these trace elements. Structural integrity: The three dimensional architecture of proteins and nucleic acids depends upon zinc and manganese as well as iron and copper. These metals bind and hold large molecules in active configurations. An example is the requirement of zinc for proper conformation of the taste- bud proteins in the tongue. Without zinc, taste and smell are lost. Iodide is incorporated into the amino acids which synthesize thyroid hormones.

Important Terms A ligand is a Lewis base that coordinates to a central metal atom or ion. A donor atom is the atom in a ligand that donate a lone pair of electrons to form a coordinate covalent bond. A unidentate ligand is a ligand that can bind through only one atom. A polydentate ligand is a ligand that can bind through more than one donor atom. There are known examples of bidentate, tridentate, quadridentate, quinquedentate, and sexidentate ligands. Chelate complexes are complexes that have a metal atom or ion and polydentate ligand(s) that form rings. The coordination number is the number of donor atoms coordinated to a metal atom or ion. A coordination sphere includes the metal atom or ion and the ligands coordinated to it. The coordination sphere does not include uncoordinated counter ions.

Important Terms For the complex compound K3[Co(CN)6] the coordination number is _________, and the coordination sphere is _______.

Rules for Naming Complex Species Nomenclature Rules for Naming Complex Species Cations (+ ions) are named before anions (- ions). Coordinated ligands are named in alphabetical order. Prefixes that specify the number of each kind of ligand (di = 2, tri = 3, tetra = 4, penta = 5, hexa = 6, etc.) are not used in alphabetizing Prefixes that are part of the name of the ligand, such as in diethylamine, are used to alphabetize the ligands. For complicated ligands, especially those that have a prefix such as di or tri as part of the ligand name, these prefixes are used to specify the number of those ligands that are attached to the central atom. bis = 2 tris = 3 tetrakis = 4 pentakis = 5 hexakis = 6 The names of most anionic ligands end in the suffix -o. Examples of ligands ending in –o are: Cl- chloro S2- sulfido O2- oxo The names of most neutral ligands are unchanged when used in naming the complex. There are several important exceptions to this rule including: NH3 ammine H2O aqua The oxidation number of a metal that exhibits variable oxidation states is designated by a Roman numeral in parentheses following the name of the complex ion or molecule. If a complex is an anion, the suffix "ate" ends the name. No suffix is used in the case of a neutral or cationic complex. Usually, the English stem is used for a metal, but if this would make the name awkward, the Latin stem is substituted. ferrate instead of ironate plumbate instead of leadate

Typical Simple Ligands Important Terms Typical Simple Ligands Ion/Molecule Name Name as a Ligand NH3 ammonia ammine CO carbon monoxide carbonyl Cl- chloride Chloro CN- cyanide cyano F- fluoride fluoro OH- hydroxide hydroxo NO nitrogen monoxide nitrosyl NO2- nitrite nitro PH3 phosphine

Nomenclature Name the following compounds: Na3[Fe(Cl)6] [Ni(NH3)4(OH2)2](NO3)2

tris(ethylenediammine) cobalt(III) nitrate Nomenclature Write formulas for the following compounds: potassium hexacyanochromate(III) tris(ethylenediammine) cobalt(III) nitrate

Structures The structures of coordination compounds are controlled primarily by the coordination number of the metal. Usually the structures can be predicted by VSEPR theory (Chapter 8). The geometries and hybridizations for common coordination numbers are summarized in this table.

CN Geometry Hybridization Example 2 Linear sp [Ag(NH3)2]+ 4 Tetrahedral sp3 [Cd(NH3)4]2+ square planar sp2d [Cu(OH2)4]2+ 5 trigonal bipyramid sp3d Fe(CO)5 Square pyramidal sp2d2 [Mn(Cl)5]3- 6 Octahedral sp3d2 [Fe(CN)6]4-

Structures Sketch the shape of the hexacyanaochromate(III) ion.

Isomerism in Coordination Compounds Isomers . two or more forms of a compound having the same composition • Structural isomers involve different atom to ligand bonding sequences. hydration isomers isomers exchange water as ligand and hydrate ionization isomer exchange ion between ligand and anion coordination isomers denote an exchange of ligands between the coordination spheres of the cation and anion. linkage isomers different ligands or different attachment of ligands • Stereoisomers (identical bonding) • geometrical isomers • optical isomers coordination sphere isomers

Structural (Constitutional) Isomers Hydrate isomers are a special case of ionization isomers in which water molecules may be changed from inside to outside the coordination sphere. For example: [Cr(OH2)6]Cl3 vs. [Cr(OH2)5Cl]Cl2. H2O vs. [Cr(OH2)4Cl2]Cl2. 2H2O Note whether the water molecule(s) are inside or outside the coordination sphere.

Structural (Constitutional) Isomers [Cr(OH2)6]Cl3 [Cr(OH2)5Cl]Cl2. H2O [Cr(OH2)5Cl]Cl2. H2O [Cr(OH2)4Cl2]Cl2. 2H2O

Structural (Constitutional) Isomers Ionization or Ion-Ion Exchange Isomers [Pt(NH3)4Cl2]Br2 compared to [Pt(NH3)4Br2]Cl2 Note where the Cl’s and Br’s are in the structures, that is what makes these two species isomers. [Pt(NH3)4Cl2]Br2 [Pt(NH3)4Br2]Cl2

Structural (Constitutional) Isomers Coordination isomers denote an exchange of ligands between the coordination spheres of the cation and anion. For example look at these two isomers: [Pt(NH3)4][PtCl6] vs [Pt(NH3)4Cl2][PtCl4] The isomeric distinction is whether the ligands are on the cation or the anion.

Structural (Constitutional) Isomers [Pt(NH3)4][PtCl6] [Pt(NH3)4Cl2][PtCl4]

Structural (Constitutional) Isomers Linkage isomerism if a ligand contains more than one atom with a free electron pair, the ligand may be bound to the central atom via the different atoms. bonding via N nitro- bonding via O nitrito- bonding via C cyano- bonding via N isocyano- bonding via S thiocyanato- bonding via N isothiocyanato-

Structural (Constitutional) Isomers [Co(NH3)5ONO]Cl2 [Co(NH3)5NO2]Cl2

Stereoisomers Stereoisomers are isomers that have different spatial arrangements of the atoms relative to the central atom. Complexes with only simple ligands can occur as stereoisomers only if they have coordination numbers equal to or greater than four. Geometrical or positional isomers are stereoisomers that are not optical isomers. Cis-trans isomers have the same kind of ligand either adjacent to each other (cis) or on the opposite side of the central metal atom from each other (trans). Note where the ligands are positioned relative to the central atom.

Stereoisomers cis- [Pt(NH3)2Cl2] trans-[Pt(NH3)2Cl2]

Stereoisomers Other types of isomerism can occur in octahedral complexes. Complexes of the type [MA2B2C2] can occur in several geometric isomeric forms: trans- trans- trans- cis- cis- cis- cis- cis- trans-

Stereoisomers trans-diammine-trans-diaqua-trans-dichlorocobalt(III) ion cis-diammine-cis-diaqua-cis-dichlorocobalt(III) ion

cis-diammine-cis-diaqua-trans-dichlorocobalt(III) ions Stereoisomers trans-diammine-cis-diaqua-cis-dichlorocobalt(III) ion cis-diammine-cis-diaqua-trans-dichlorocobalt(III) ions

cis-diammine-trans-diaqua-cis-dichlorocobalt(III) ions Stereoisomers cis-diammine-trans-diaqua-cis-dichlorocobalt(III) ions

Octahedral complexes can exhibit another type of geometric isomerism - mer-fac isomerism. mer isomerism involves all three similar ligand lying in the same plane, or meridianl like a globe. fac facial involves a grouping of three similar ligands that are arranged on a triangular face of the octrahedron fac and mer-Co(NH3)3Cl3

Stereoisomers Optical isomers are mirror images of each other that are not superimposable. The cis-diammine-cis-diaqua-cis-dichlorocobalt(III) ion has two different forms called optical isomers or enantiomers. Separate equimolar solutions of the two isomers rotate plane polarized light by equal angles but in opposite directions. The phenomenon of rotation of polarized light is called optical activity.

cis-diammine-cis-diaqua-cis-dichlorocobalt(III) ion Stereoisomers These are the optical isomers of: cis-diammine-cis-diaqua-cis-dichlorocobalt(III) ion