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
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.
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)
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.
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.
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.
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.
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.