COORDINATION COMPOUNDS COMPLEX By s. r. ratnam

Alfred Werner Switzerland University of Zurich Switzerland b d Alfred Werner ( ) 1893, age 26: coordination theory Nobel prize for Chemistry, 1913 Addition of 6 mol NH 3 to CoCl 3 (aq) Conductivity studies Precipitation with AgNO 3

Werner’s explanation of coordination complexes Metal ions exhibit two kinds of valence: primary and secondary valences The primary valence is the oxidation number (positive charge) of the metal (usually 2+ or 3+) The secondary valence is the number of atoms that are directly bonded (coordinated) to the metal The secondary valence is also termed the “coordination number” of the metal in a coordination complex

Werner Coordination Theory CompoundMoles of ionsMoles of AgCl(s) “CoCl 3. 6NH 3 ” “CoCl 3. 5NH 3 ” “CoCl 3. 4NH 3 ” “CoCl 3. 3NH 3 ” Cl – attached to NH 3 may be dissociated

Werner Coordination Theory CompoundMoles of ionsMoles of AgCl(s) [Co(NH 3 ) 6 ]Cl 3 [Co(NH 3 ) 5 Cl]Cl 2 [Co(NH 3 ) 4 Cl 2 ]Cl [Co(NH 3 ) 3 Cl 3 ] Proposed six ammonia molecules to covalently bond to Co 3+

Coordination Chemistry Definitions Coordination compounds – compounds composed of a metal atom or ion and one or more ligands (atoms, ions, or molecules) that are formally donating electrons to the metal center Miessler, Tarr, p. 278

Coordination Chemistry Definitions Coordination compounds 3Cl – M ligand N forms a coordinate covalent bond to the metal (coordination sphere) (counterion)

Coordination Chemistry Definitions Ligands – simple, ‘complex’ Denticity – different number of donor atoms Chelates – compounds formed when ligands are chelating (Gk. crab’s claw) M bidentate

Valence Bond Theory Developed by Linus Pauling

Overlap of an empty orbital with a fully- filled orbital leads to the formation of a co-ordinate covalent bond or dative bond Bonding in Coordination Compounds Valence Bond Theory

Tro, Chemistry: A Molecular Approach 15 Geometry of complex Magnetic properties of complex Electronic configuration of Metal ion Nature of Bonding VBT explains

VBT Valence Bond Theory

Tro, Chemistry: A Molecular Approach 20 Geometries in Complex Ions

tetrahedral

octahedral

Polydentate Ligands Ethylenediaminetetraacetate, mercifully abbreviated EDTA, has six donor atoms.

Valence Bond Theory Metal or metal ion: Lewis acid Ligand: Lewis base Hybridization of s, p, d orbitals C.N.Geometry 4tetrahedral Hybrids sp 3 square planardsp 2 trigonal bipyramidaldsp 3 or sp 3 d octahedrald 2 sp 3 or sp 3 d 2

Valence Bond Theory Example 1: [Co(NH 3 ) 6 ] 3+ Co [Ar] 3d 7 4s 2 Co 3+ [Ar] 3d 6 3d3d4s4s4p4p if complex is diamagnetic 4d4d d 2 sp 3 octahedral :

Valence Bond Theory Example 2: [CoF 6 ] 3– Co [Ar] 3d 7 4s 2 Co 3+ [Ar] 3d 6 if complex is paramagnetic 3d3d4s4s4p4p4d4d 4sp 3 d 2 octahedral

Valence Bond Theory Example 3: [PtCl 4 ] 2–, diamagnetic Pt 2+ [Xe] 4f 14 5d 8 5d5d6s6s6p6p dsp 2 square planar

Valence Bond Theory Example 4: [NiCl 4 ] 2–, tetrahedral Ni 2+ [Ar] 3d 8 3d3d4s4s4p4p 4sp 3 paramagnetic

Valence Bond Theory Ligands (Lewis base) form coordinate covalent bonds with metal center (Lewis acid) Relationship between hybridization, geometry, and magnetism Inadequate explanation for colors of complex ions e.g., [Cr(H 2 O) 6 ] 3+, [Cr(H 2 O) 4 Cl 2 ] +