Coordination Chemistry: Ligand Field Theory Roat-Malone Ch. 1 (P. 1-22) Prof. Arthur D. Tinoco University of Puerto Rico, Rio Piedras Campus Chemistry 8990 Semester 1 2014-2015

The d Orbitals dx2-y2 dxz dz2 dyz dxy

d Orbital Energy Levels in different Coordination Geometries

4. Coordination Numbers and Structures A. Rules of thumb- Factors favoring low coordination numbers: Soft ligands and metals in low oxidation states Large bulky ligands Counterions of low basicity “Least coordinating anion” BArF

4. Coordination Numbers and Structures A. Rules of thumb- Factors favoring high coordination numbers: Hard ligands and metals in high oxidation states Small ligands Large nonacidic cations

4. Coordination Numbers and Structures I. Low coordination numbers Making bonds makes things more stable and so low coordination numbers are not favored. Coordination number = 1 Rare for complexes in condensed phases (solids and liquids). Often solvents will try to coordinate.

4. Coordination Numbers and Structures ii. Coordination number = 2 Also rare Ag(NH3)2+; d10 metal Linear geometry iii. Coordination number = 3 [Au(PPH3)3]+; d10 metal Trigonal planar geometry

4. Coordination Numbers and Structures II. Coordination Number = 4 Avoid crowding large ligands around the metal. Tetrahedral geometry is quite common Favored sterically Favored for L = Cl-, Br-, I- and M = noble gas or pseudo noble gas configuration Ones that don’t favor square planar geometry by ligand field stabilization energy. Square planar Ligands 90° apart d8 metal ions; M(II) Smaller ligands, strong field ligands that π-bond well to compensate for no six-coordination Cis and trans isomers

4. Coordination Numbers and Structures III. Coordination Number = 5 Trigonal bipyramidal vs square pyramidal Highly fluxional Isolated complexes tend to be a distorted form of one or the other D3h C4v TBP Geometry favored by: d1, d2, d3, d4, d8, d9, d10 metal ions Electronegative ligands prefer axial position Big ligands prefer equatorial position Sq Pyr Geometry favored by: d6 (low spin) metal ions

4. Coordination Numbers and Structures IV. Coordination Number = 6 Mostly octahedral geometry (Oh) Favored by relatively small metals Isomers ii. Distortions from Oh Tetragonal distortions: Elongations or compressions along Z axis Symmetry becomes D4h

4. Coordination Numbers and Structures Trigonal distortions (Elongation or compression along C3 axis) Trigonal prism (D3h) Favored by chelates with small bite angles or specific types of ligands Trigonal antiprism (D3d) Rhombic distortions (Changes in two C4 axes so that no two are equal; D2h)

4. Coordination Numbers and Structures V. Coordination Number = 7 Not common Pentagonal bipyramid Capped octahedron 7th ligand added @ triangular face Capped trigonal prism 7th ligand added @ rectangular face

4. Coordination Numbers and Structures VI. Coordination Number = 8 Not common Cube CsCl Trigonal dodecahedron Square antiprism

4B. Transition Metal Coordination Geometrics most relevant in Biological Systems