L2: Phosphines, N-heterocyclic carbenes, ligand substitution

Question 1

Phosphine ligands (about)

Answer 1

• PR₃, common spectator ligands
• Steric and electronic properties can be tailored via R group
• Lone pair on central P allows sigma-donation
• Can also act as pi-acceptors (dep. on R grp)
• Many are very smelly, oxygen-sensitive liquids (except triarylphosphines)

Question 2

Phosphine ligand examples ordered b increasing pi-acceptor ability

Answer 2

• PMe₃
• PAr₃
• P(OMe)₃
• P(OAr)₃
• PCl₃
• PF₃ approx. = CO
• Electron donating ability can be determined using relative v(CO) IR stretching frequencies

Question 3

Phosphine ligands as pi-acids

Answer 3

• Two of the 3d orbitals on the P, whilst theoretically good candidates for the pi-backbonding interactions, are actually too high lying and diffuse
• It is accepted that they are not significantly involved in pi-accepting
• It is the sigma* orbitals of the P-R bonds that accept electron density from the metal
• As R gets more electronegative, these orbitals become less high-lying and more localised on P; increasing pi-acceptor ability

Question 4

In coordination, how is P-R bond length affected and why?

Answer 4

• In coordination, the effect of added P-R sigma* density weakening the P-R bond and the sigma-donation of the lone pair on P to an empty metal orbital strengthening the bond effectively cancel
• There is little to no change to P-R bond length on coordination

Question 5

Steric effects of R groups (phosphine ligand)

Answer 5

• Varying size of R groups determine cone angle
• Cone angles are calculated using space filling models of the M-PR₃ group with the M-P bond length set at 228 ppm
• Cone angle is the angle subtended at the metal that is just big enough to contain all the PR₃ ligand

Question 6

Bite angle in chelating phosphines

Answer 6

• Bite angle varies between different bidentate ligands- It is the preferred P-M-P bond angle set by the ligand ‘backbone’ groups linking the P donors
• Denoted ‘beta_n’
• The observed bite angle may deviate from the natural bite angle as a result of compromise with the stereochemical requirements of the metal or the other ligands

Question 7

Kappa notation

Answer 7

• In bidentate or polydentate ligands, kappa denotes how many donor atoms of that ligand are bonded to the same metal centre

Question 8

N-heterocyclic carbenes (about)

Answer 8

• NHCs; used as 2-e^- ligands
• First found by deprotonation of an imidazolium salt (by treatment w/ a strong base)
• The steric protection of the adamantyl was found not to be a prerequisite for NHCs, many are accessible
  e.g. IMes, IPr, IBut^t, SIMes
• Their electronic effects are not as variable as those of phosphine ligands

Question 9

Bonding in NHCs

Answer 9

• NHCs are electron rich and good, neutral, 2-electron sigma-donor ligands. They form a string bond with many t.metals (good spectator ligands, offering steric protection)
• Poor pi-acceptors; the empty 2p^z orbital of the carbene is pushed to high energy by interaction w/ the 2 ‘lps’ on adjacent, planar sp²-hybridised, N atoms

Question 10

Examples 1,2, 3 and 4-electron donor ligands (single donor atom)

Answer 10

• 1: -Halogens, -H, -CN, -OR, -NR₂, bent nitrosyl (X-type)
• 2: CO, PR₃, P(OR)₃, CNR, :CR₂ (L-type)
• 3: -OR, -NR₂, -CR, -N=O (linear nitrosyl)
• 4: -NR (imido)
• alkoxide and amide can function as 1 or 2 electron donors, due to additional pairs on donor atom; initially count as 1 unless they are bridging 2 electron centres

Question 11

Electron counting for etaⁿligands/pi complexes

Answer 11

• Number of electrons contributed is equal to the hapticity of the ligand
• 18 electron rule can be used to predict the hapticity of a polyene or polyenyl ligand

Question 12

Determining oxidation state

Answer 12

• Separate from electron counting
• Formal charge on metal when all ligands are separated from the metal as lewis bases
• Ligands that donate odd no. electrons are considered as anions (except NO)
• Ligands that donate an even no. are considered as uncharged

Question 13

Ligand substitution (about the two types)

Answer 13

• Either dissociative or associative (dep. on electron count of complex undergoing substitution; always dissociative for 18-electron complexes)
• Dissociative: complex loses ligand (slow), forms species w/ vacant coordination site, incoming ligand rapidly adds. For octahedral, ligand stereochemistry can be retained or lost dep. on whether 16-electron IM remains square pyramidal. Can still occur w 16-electron complexes, particularly if there’s steric crowding at metal centre
• Associative : rate limiting step is ligand addition, (typically w/ coordinatively unsaturated complexes), followed by rapid dissociation of one of the original ligands. Very rarely observed in 18-electron complexes (e.g. nitrosyls and NO since they readily change coordination type)

Question 14

Effect of bulky ligands in substitution reactions

Answer 14

• Facilitates dissociative substitution
• Also stabilises coordinatively unsaturated species