Coordination Chemistry General aspects Ch 9 Bonding Ch

Coordination Chemistry • General aspects (Ch. 9) • Bonding (Ch. 10) • Electronic spectra (Ch. 11) • Reaction mechanisms (Ch. 12)

Acids and bases (the Lewis concept) A base is an electron-pair donor An acid is an electron-pair acceptor acid adduct base Lewis acid-base adducts involving metal ions are called coordination compounds (or complexes)

Coordination complexes Coordinated ligands Central metal atom Inner coordination sphere counteranion Inner coordination sphere The metal cation is the Lewis acid, the ligands are the Lewis bases

Naming coordination complexes

General nomenclature rules in coordination chemistry 1. Cation first, then anion (as for simple salts) (K 3[Fe(CN)6], potassium hexacyanoferrate) 2. Inner coordination sphere in square brackets in formula. Ligands named before the metal 1. Hexaaminecobalt(III) chloride: [Co(NH 3)6]Cl 3 3. Number of ligand indicated by prefix (di, tri, tetra or bis, tris, tetrakis if ligand in parenthesis) 4. tris(bipyridine)iron(II) chloride: [Fe(bipy)3]Cl 2 5. Ligands named in alphabetical order ignoring prefix 6. Anionic ligands are given the suffix -o (chloro-, sulfato-, nitrato-) while neutral ligands retain name (except aqua for H 2 O and ammine for NH 3) 6. Metal named after ligands with oxidation state in roman numerals or give overall charge of 7. coordination sphere Fe(III), tetrachloroplatinate(-2) 7. Cis (adjacent)-trans (opposite) or fac (C 3 v) –mer (C 2 v) isomers are indicated with prefix 8. Bridging ligands are indicated with m (greek mu) 9. m-oxo for M-O-M 10. If complex is anionic, use ending “-ate” 11. -cobaltate, ruthenate, but note ferrate for Fe, argentate for Ag, plumbate for Pb, 12. stannate for Sn and aurate for Au

Isomerism • Stereoisomers (enantiomers, diastereomers, cis/trans, mer/fac, conformational) have same metal ligand bonds but different 3 D arrangement. • Hydrate (solvate) isomers, ionization, linkage, coodination isomers have different metal-ligand bonds.

Examples of Four Coordinate Stereoisomers planar stereoisomers trans cis Tetrahedral, chirality now possible. Four different monodentate ligands.

Chirality in tetrahedral complexes Very common (2 enantiomers if all ligands different)

Examples of Six coordinate Stereoisomers How many stereoisomers are there of formula Mabcdef? For the six sites in the octahedron there are 6! = 6 * 5 * 4 * 3 * 2 * 1 ways of positioning the ligands. However some of these ways are the same structure; simply rotated. An octahedron has many rotations which simply interchange ligands: 8 C 3, 6 C 2, 6 C 4 and 3 C 2. Thus there are 23 rotated structures to be generated from an original structure. 6!/(23+1) = 30 stereoisomers. For some complexes with multidentate ligands there are geometry constraints which reduce the number of isomers.

Chirality in octahedral complexes

Multidentate ligands and isomer count. Let AA be a multidentate ligand which must bond cis. For octahederal complex MAAbcde how many stereoisomers? Permutation count is not 6! but 6 * 4 *4! # stereoisomers = 6 * 4 *4!/(24*2) Only four spots for the second A to enter. For a complex MAABCde Due to rotations Since both ends of the AA are the same.

Rotation factor Due to A-A symmetric ligand For a complex MAABCde Number of stereoisomers = 6 * 4 * (2 *2 * 2! + 2*3 *2!)/(24 * 2) = 10 stereoisomers Assign first A and second A in cis position

Chirality in octahedral complexes with chelating ligands

Several chelate rings and chirality D isomer L isomer Right hand screw Left hand screw

Conformational Isomers The chelate rings can have alternative conformations.

Constitutional Isomers • Hydrate Isomers: in crysal structure is water part of the first ligand shell or a hydrate – – [Cr(H 2 O)6]Cl 3, violet [Cr. Cl(H 2 O)5]Cl 2. H 2 O, blue-green [Cr. Cl 2(H 2 O)4]Cl. 2 H 2 O, dark green [Cr. Cl 3(H 2 O)3]. 3 H 2 O, yellow green

Constitutional Isomers • Hydrate Isomers: in crysal structure is water part of the first ligand shell or a hydrate – – [Cr(H 2 O)6]Cl 3, violet [Cr. Cl(H 2 O)5]Cl 2. H 2 O, blue-green [Cr. Cl 2(H 2 O)4]Cl. 2 H 2 O, dark green [Cr. Cl 3(H 2 O)3]. 3 H 2 O, yellow green • Ionization isomerization: different ions produced in solution – [Co(NH 3)5 SO 4]NO 3 & [Co(NH 3)5 NO 3] SO 4

• Coordination Isomers: More than one ratio of ligand can exist but maintaining overall ratio – [Pt(NH 3)2 Cl 2] – [Pt(NH 3)3 Cl] [Pt(NH 3)Cl 3] • Linkage (ambidentate) isomerism – Thiocyanate, SCN-, can bind through either the N (to hard acids) or through S (to soft acids). – Nitrite, NO 2 -, can bond through either the N or the O

Typical coordination numbers and structures of coordination complexes and isomerism

Coordination number 1 Very rare, bulky ligands, linear structures, no possible isomers

Coordination number 2 Also rare, typical of d 10, linear structures, no possible isomers

Coordination number 3 Also typical of d 10, trigonal planar structures (rarely T-shaped), no possible isomers

Coordination number 4 Very common Tetrahedral (2 enantiomers if all ligands different) Square planar (2 geometrical isomer for two types of ligands) typical of d 8

Tetrahedral Square planar

Coordination number 5 Trigonal bipyramidal (tbp) Square-based pyramidal sbp) Very similar energies, they may easily interconvert in solution (fluxionality)

Coordination number 6 Octahedral most common Trigonal prism less common

Some possible isomers in octahedral complexes cis-MA 2 B 4 trans-MA 2 B 4 fac-MA 3 B 3 mer-MA 3 B 3

Some examples of trigonal prismatic structures

Coordination number 7 Pentagonal bipyramidal Capped octahedral Capped trigonal prismatic

Examples of coordination number 7