Phosphine Ligands

Question 1

Synergic bonding

Answer 1

σ-donation from PR3 HOMO to vacate eg M AO
π-back donation from occupied t2g M AO to P-C σ* AO

Question 2

Consequences of synergic bonding

Answer 2

Phosphine σ-donates electron density onto MC = more electron rich = increased π-back donation = increased occupation of 2π* = weaker CO = decreased νCO

Question 3

Trends in νCO

Answer 3

Increased substituent electronegativity = PR3 poorer net donor so νCO decreases

Question 4

Steric effects

Answer 4

Quantified by cone angle (θ), measured from scale models, θ increases with increasing R group size
P(OR)3 = extra P-O bond removes R group further from metal = decreases θ compared to corresponding phosphine

Question 5

Chemical consequences of bulky phosphines

Answer 5

Dissociate easier and stabilise unsaturated species = steric protection