Structure of Organometallic Complexes

1. Structure of Organometallic Complexes • Thus far we have only considered bonding modes between a metal centre and various ligands. • What knowledge do we have to interpret/predict the structure of organo-transition metal complexes? • What structural features are needed for a complex to exhibit catalytic activity? • Properties of the metal centre: • Electronic configuration (orbital occupancy) • Oxidation state • Ionic potential and polarization • Properties of the ligands: • Structure, polarizability, size, basicity • Properties of the complex: • Effective Atomic Number (EAN) • Overall charge and counterion structure

2. Oxidation States and Electron Counting • The oxidation state of a metal centre represents its formal charge remaining after ligands are “removed” in their closed shell configurations. • Convention is that when a ligand is removed from the complex, it takes its electrons with it. • The oxidation number of the metal relates to: • ionic potential (q/r) in electrostatic interactions • reactivity of the complex • Oxidation numbers have little meaning for many organometallic complexes, given that the charge on each ligand is arbitrarily assigned • it may bear little resemblance to the actual electron distribution in a complex.

3. Calculating the Oxidation State • To calculate the oxidation state of the metal, sum the formal charges of all ligands to which it is coordinated. • Note that the formal charge of some ligands depends on its mode of bonding (see O2, olefins)

4. Electron Counting Scheme for Transition Metals • The number of electrons formally assigned to the metal centre depends on atomic number as well as oxidation state.

5. Effective Atomic Number: 18 Electron Rule • The effective atomic number is derived from valence bond theory, where ligand coordination allows the metal centre to reach a noble gas configuration through covalent bond formation. • Complexes of the transition metals with p-acid ligands, as well as their organometallic complexes, generally obey the EAN rule. • Their stoichiometries and molecular structures usually can be predicted as arising from a tendency to surround each metal with a full complement of 18 electrons. • The nd, (n+1)s and (n+1)p orbitals are all valence orbitals, and all their bonding capacity is used when the 18 electron configuration is reached. • Note that charge effects and ligand size are important as well as available bonding orbitals • Coordinatively unsaturated complexes are those with fewer than 18 electrons. Many of the 14 electron and 16 electron complexes we encounter in catalytic processes are reactive.

6. Effective Atomic Number

7. Coordination Number and Geometry • The geometry of coordination complexes has been discussed in CHEM 312. • Coordination numbers depend on: • number of electrons about the metal centre (max 18) • ionic potential of the metal • steric interactions between ligands • Note that specific geometries are assigned from crystal structures (solid state). The structure of active catalytic compounds cannot be measured, but proposed from our knowledge of stable compounds.

8. Naming Organo-Transition Metal Complexes • The names of neutral ligands are usually unchanged (H2O aquo, NH3 ammine, CO carbonyl and NO nitrosyl are exceptions). • The names of coordinated anions always end in -o. Those free anions that end in -ide are changed to -o. Those which end in -ate or -ite change to -ato or -ito. • Ligands are listed first in alphabetical order, followed by the name of the central atom and its oxidation state as a Roman numeral • there is no ending modification in neutral or cationic complexes, while in anionic complexes, the name of the metal is modified to an -ate ending • The number of a given ligand is indicated by di-, tri-, tetra- and so on if it is monatomic (Cl-) or a neutral ligand with a special name (CO) • Polyatomic ligands (such as PPh3) are placed within parentheses and prefixed by bis (2), tris (3), tetrakis (4) and so on.

9. Stoichiometric Reactions of Organometallic Complexes • Catalytic reactions of organometallic complexes are described in terms of sequences of stoichiometric reactions. These often include: • Ligand Coordination Ligand Dissociation • Oxidative Addition Reductive Elimination • Insertion b-elimination • with transmetallation, metathesis and coupling/cleavage reactions appearing less frequently.