Hydroformylation Flashcards
What is hydroformylation (draw example)?
(Basic Reaction)
- Production of aldehydes from aklenes (adding CHO and H atom to form C=C)
- Important in industrial purposes (aldehydes often hydrogenated to alcohols) (purposes are cobalt catalysis and rhodium catalysis)
What is the range of transition metal electronegativities?
1.3-2.3
Why is assigning a oxidation number (notional charge) not entirely correct?
The charge is not entirely on the more electronegative atom. The electrons are shared.
ie. NbCl5 has Nb=+5 but reality is chlorine shares some electrons so Nb will be less than +5
What is the d^n number?
- Number of valence electrons left on the metal after taking account of the oxidation state of the metal
1st row transition metal:
[Ar] 4s^2 3d^x - Although the 4s level is filled before the 3d, when electrons are removed they are taken from the 4s level before the 3d level
e.g. dn number of Fe(II) : - Fe(0) = [Ar] 4s2 3d6
- Fe(II) loses 2 electrons from the 4s level - Fe(II) therefore has a d6 configuration - If metal in a complex: assume all valence electrons are in d-level
e.g. dn number of iron in Fe(CO)5: - oxidation state of Fe = 0
- Fe = [Ar] 4s2 3d6 but in a complex
- all electrons in d-level, = d8 configuration
How to find electron count?
- Identify the valence electrons of the metal (determine d^n number)
- Find out how many electrons each of the ligands donates:
- monodentate Lewis base ligands (e.g. NH3 or PPh3) contribute two electrons (a pair)
- simple anionic ligands like Cl–, CN–, H– and Me– each donate two electrons
- π-ligands (e.g. ethene) donate according to the number of p-electrons associated with the metal
e.g. C2H4 will donate 2 electrons ; benzene (C6H6) will donate 6 (normally…)
(chloro and methyl are regarded as Cl– and Me–. Remember, when applying the oxidation state formalism we notionally transferred the electrons to the chloride or Me group, so for
electron counting purposes they are regarded as anionic ligands)
NOTE: remember that CH3– is isoelectronic with NH3 : both have a (lone) pair of electrons
What is the coordination number (CN)?
- Number of simple 2-electron ligands attached to a metal
* simple for ligands like NH3, Cl– and extension to polydentate chelating ligands logical
* more complicated for organometallic compounds with large p-ligands, leading to
ambiguity and reducing the use of this classification
Generally:
- ethene regarded as occupying ONE coordination site
- benzene regarded as occupying THREE coordination sites (donates 6 electrons…)
What is coordination geometry?
- Relates to number of ligands and the d^n number
What does CN=6 indicate?
Almost all octahedral (exception = WMe6 which is trigonal prismatic)
What does CN=5 indicate?
Trigonal bipyramidal (tbp) or square pyramidal
* both often close in energy and therefore difficult to predict geometry
* steric factors favour trigonal bipyramid but electronic factors may ‘tip the balance’ in favour of square pyramidal
* general rule: d8 = tbp (e.g. Fe(CO)5) d6 = square pyramidal
What does CN=4 indicate?
Tetrahedral or square planar
* steric factors always favour tetrahedral whereas (again) electronic factors for certain dn configurations can favour square planar
* general rule: d10 = always tetrahedral
d8 = (of heavier transition elements 4d and 5d) always square planar
NOTE: that electronic ligand field energies are greater for the 2nd and 3rd transition series than the 1st. For a light d8 metal like Ni(II) the situation is finely balanced and both geometries may be found according to whether steric or ligand field energies dominate
What are the properties of Lewis Acids and Bases?
- Generally, ligands act as Lewis bases: they donate electron pairs to the metal, which therefore acts as a Lewis acid
- Some ligands are readily identifiable as Lewis bases
e.g. NH3 = Lewis base because of the lone pair of electrons on nitrogen
(the methyl ligand [CH3]– is isoelectronic with ammonia and acts in exactly the same way) - However, some ligands which bind very strongly to transition metals are not readily recognised as Lewis bases
- Carbon monoxide is a very weak Lewis base except when interacting with transition metals
(similarly, benzene and ethene not usually be regarded as having significant Lewis base properties but can bind effectively to transition metals due to synergic bonding)
Explain hard and soft acids and bases
- Lewis bases with a small electronegative donor atom (e.g. N, O, Cl, F) have tightly held non-polarizable lone pairs and are termed “hard bases”
- Ligands with larger, less electronegative donor atoms have more diffuse and polarizable lone pairs and are referred to as “soft bases” (e.g.PPh3, CO, H–, Me–)
Similar concepts apply to the acceptor orbitals of Lewis acids: - For the transition metals, Lewis acid softness increases left to right across the Period, and down the Groups
- Also the lower the oxidation state of the metal, the softer it acts as a Lewis acid:
- hence Pt(II) is softer than Ni(II); Pt(0) is softer than Pt(II)
- the hard extreme is Ti(IV)
Soft acids bind most effectively to soft bases and hard acids bind best to hard bases
NOTE: this preference does not go as far as excluding hard/soft interactions
How are distinctions made between hard and soft acids and bases?
“classical” transition metal coordination compounds (involve predominantly hard ligands)
soft ligands are high on the spectrochemical series and produce large ΔO splitting
- therefore the eg* orbitals are relatively high in energy and usually remain unoccupied
What types of ligands are σ-donor ligands?
- some ligands have only a single pair of donor electrons in a σ-type orbital
- the interaction involves simple donation from the ligand to a vacant metal orbital
e.g. NH3 CH3– H–
What types of ligands are π-donor ligands?
- some ligands (e.g. halides) have other electrons as well as those used to form a π-bond * if these unused electrons are in an orbital of p-symmetry they may interact with the dxy dxz or dyz orbitals (lie between the axes, and have matching p-symmetry)
- the filled π-orbitals on the ligand can donate electron density to the dp orbital on the metal
NOTE: remember they also act as s-donors
What are features of carbonyl ligands (σ*-orbital)?
- the C and O atoms joined by a triple bond (σ plus 2 x π)
- also at lower energy = a filled σ*-orbital which has a lobe located on C and is oriented away from the oxygen
- acts as a s-donor orbital towards the metal
- electron density is transferred from the ligand to the metal
What are features of carbonyl ligands (π*-orbital)?
- the C and O atoms joined by a triple bond (σ plus 2 x π)
- associated with the CO p-bonding orbital there is an empty π*-antibonding orbital
- this has lobes which extend beyond the C atom and are of the correct symmetry to interact with filled dxz orbitals on the metal
- electron density is transferred back from the metal to the ligand (mecahnism called: back-bonding)
- strengthens the overall metal ligand bond and the two bonding
mechanisms are referred to as “synergic bonding”
Draw a diagram of back-bonding
Draw the MO diagram for CO and discuss it’s significance
Draw the MO diagram for CO with electrons
