Metal-alkyne complexes

Question 1

Alkynes as ligands

Answer 1

2 electron donors (when bound in the pi form)

Tend to ligate to electron-rich metals

Question 2

Main binding modes of alkynes

Answer 2

Monodentate (formal triple bond with slightly reduced electron density)
Bidentate (formal double bond due to removal of so much electron density)

sp3 bound is also possible (sigma rather than pi bonding)

Question 3

Spectroscopic data for a free alkyne

Answer 3

Free C—C = 120-134 pm
IR 2190-2260 cm^-1

Can use these bond metrics to determine the binding mode of an alkyne

Question 4

What does bending of alkyne substituents away from the plane suggest?

Answer 4

Greater interaction between metal and ligand

Question 5

Uses of substitution reactions

Answer 5

Can be used to stabilise otherwise explosive halo-alkynes e.g. Cl-C—C-Cl
Can be used to prepare an otherwise unknown cyclohexyne (would never exist in isolation)

Question 6

Main methods for the preparation of metal-alkyne complexes

Answer 6

1. Substitution = kicking out a labile ligand OR forming a more stable chelate/oxidation state/coordination number
2. Elimination to form benzyne

See flashcards

Question 7

Transforming alkyne into allyl

Answer 7

Draw

Question 8

When do alkynes act as 4 electron donors?

Answer 8

With electron poor metals (generally in high oxidation state)
Probably also need a low coordinate complex so there is enough space for the alkyne

Possible to form by replacing 2 x CO (2 x 2 e) in a carbonyl complex and generating an 18 VE electron complex

Question 9

How else can alkynes act as 4 electron donors?

Answer 9

They can bridge 2 metal centres and donate 2 e per metal

Alkyne bond order reduced to formal C-C single bond (alkane-type bonding mode)

Question 10

Metal-ligand orbital interactions for alkynes as 4 electron donors

Answer 10

Draw

Question 11

Reactions of alkyne complexes

Answer 11

Cyclisation of alkynes
Incorporation of CO
Pauson-Khand reaction