Lecture 1 Flashcards
Organoelement
= semi-metals (or non-metals) whose chemistry closely resembles that of true metals
e.g. B, Si, P, As, Se, Te
Properties of transition metals
TMs have access to a large number of valence orbitals (ns, np and nd) so can therefore engage in multiple bonding with the organic ligands
Partial occupation of valence orbitals leads to donor and acceptor properties
Uses in organometallic catalysis due to ability to change coordination number and the lability of the M-C sigma-bond
Class I TM complexes
12-22 valence electrons (i.e. 18 electron rule is ignored)
3d metals
Octahedral and tetrahedral geometries
t2g non-bonding
eg* weakly anti-bonding (but can be occupied by certain ligands because 18e rule not obeyed)
Ligands = weak field, high spin, good sigma-donors
Class II TM complexes
16, 17, 18 valence electrons (always 18 electrons or less, 18e never exceeded)
Generally specific for 4d and 5d metals
t2g essentially non-bonding
eg strongly anti-bonding (unoccupied)
DeltaOct/Tet larger
Ligands = strong field, low spin, moderate sigma-donors
Class III TM complexes
18 valence electrons exactly (unless sterics are prohibitive)
t2g bonding (due to ligand interactions)
eg* strongly anti bonding (unoccupied)
DeltaOct even larger
Ligands = really strong field, very low spin, pi-acceptors
Reasons for reactions to occur
Enthalpy (salt formation, precipitation)
Entropy (gas formation, no. product molecules > no. reagent molecules)
Valence e count (18e is stable)
Formation of a stable oxidation state [e.g. Pd(0) and Pd(II) more stable than Pd(IV), which is more stable than Pd(I) and Pd(III)]
Relief of strain
Chelate formation
Methods for preparation of metal-olefin complexes
Substitution Addition Hydride abstraction Reduction Metal atom/ligand vapour co-condensation
Counter anions in TM OM complexes
AlCl4- = good for stabilising TM OM complexes
(AlCl3 can assist in halide abstraction from a metal centre in substitution reactions)
BF4- = non-coordinating counter ion
Photochemical substitution reactions
UV light can lead to loss of CO from metal centre
Metal-induced isomerisation
Metal carbonyl complex + non-conjugated diene + heating = double bond isomerisation to form conjugated diene and ligation of metal complex to form chelate (eta4)
Addition reactions to form metal-olefin complexes
Requires a coordinatively unsaturated metal
Hydride abstraction reactions to form metal-olefin complexes
Hydride abstracted from alkyl ligand by base
Reduction reactions to form metal-olefin complexes
Generally of the form M^n+ —> M^(n-2)+
AlR3 e.g. AlMe3 = reducing agent
COD
Cyclooctadiene
Not conjugated but can form chelate complexes because it can ‘hook’ round and form a bidentate complex
Metal atom/ligand vapour co-condensation reactions to form metal-olefin complexes
Generally not used anymore - only produces a homo-ligated compound (no other ligands apart from olefin), so reactivity is not selective/can’t be tuned (no differentiation between sites)
Metal in gas phase
