Metal-Hydride complexes

Question 1

Describe what metal-hydride complexes are? Any pi-back bonding? Thermal stability?

Answer 1

Metal-hydride complexes contain metal-hydrogen bonds (M-H) characterized by σ-bonding only, without M-to-L π-back-bonding.

Thermal stability increases down a group of
- 1st < 2nd and 3rd TM complexes

Question 2

How are metal hydrides synthesized?

Answer 2

Metal hydrides are synthesized through oxidative addition, reactions with bases, and protonation​​ (also oxidative addition)

Question 3

How are metal hydrides characterized?

Answer 3

Difficult to locate using X-ray diffraction
- due to low X-ray scattering power of H

Neutron scattering - need neutron source

1H NMR chemical shift is -ve

IR also used but requires isotopic labelling to distinguish M-H from CO stretches​​.

Question 4

What affects the acidity of metal hydrides?

Answer 4

Sigma donors decrease acidity

EWG ligands increase acidity - polarising the M-H bond

1st > 2nd > 3rd row

Question 5

Describe a key reactivity feature of metal hydrides.

Answer 5

Metal hydrides can undergo insertion reactions with olefin
- generating a metal-alkyl
- insertion of CO into M-H is almost unknown​​.

Question 6

What are metal-alkyl complexes?

Answer 6

Metal-alkyl complexes involve a TM-alkyl bond.

Historically believed weak, this bond is now known to be strong and essential in organometallic chemistry​​.

Question 7

How does the bond strength of TM-alkyls vary? What’re they prone to?

Answer 7

TM-alkyl bond strength increases down a triad
- Pt > Pd > Ni

However, they are prone to decomposition via β-hydride elimination​​.

Question 8

How are metal-alkyls synthesized while avoiding β-hydrogen elimination?

Answer 8

Use bulky ligands to prevent agostic interactions

Use cyclic alkyl ligands to tie beta-Hydride back

Avoid vacant coordination sites

Avoid square planar (16 electron) complexes​​.

Question 9

What roles do metal-alkyl complexes play in synthetic processes?

Answer 9

Intermediates in C-C bond formation

Insertion/migration reactions

Polymerization

Reductive elimination​​

Reactions with electrophiles

Question 10

What is the pKa range for transition-metal hydrides?

Answer 10

TM hydrides have pKa values ranging from 1 (strong acid) to 14 (strong base) .

Question 11

How does blocking β-H elimination pathways affect the stability of TM-alkyl?

Answer 11

Blocking β-H elimination pathways INCREASES the stability of TM-alkyl complexes .

Question 12

What is an agostic interaction in metal-hydride complexes?

Answer 12

Agostic interaction refers to the stabilization of an electron-deficient metal centre through coordination of a C-H bond
-often an intermediate in β-hydride elimination.

Question 13

How is {PtMe3I}4 synthesized and what does it illustrate about TM-alkyl bonds?

Answer 13

{PtMe3I}4 was prepared in 1907, demonstrating the strength of M-C bonds

Disproves theories that TM-C bond was weak

Question 14

What is β-hydride elimination in the context of metal-alkyl complexes?

Answer 14

β-Hydride elimination involves the migration of a hydrogen atom from a β-carbon to the metal, leading to the formation of an alkene and a hydride complex. This is a common decomposition pathway for metal-alkyls .

Question 15

What is the impact of alkyl migration/CO insertion on the oxidation state of metal complexes?

Answer 15

Alkyl migration or CO insertion generates a metal-acyl complex

It decreases the metal’s electron count by 2 - 2 ligands become 1 (M-CO + M-R becomes M-C(O)R)

Ligands must be cis for alkyl migration
- start from 18e- complex
- oxidation state is unchanged

Question 16

What does the transition state for β-hydride elimination look like?

Answer 16

In β-hydride elimination, the transition state involves an agostic interaction where a C-H bond coordinates to an electron-deficient metal center, facilitating the transfer of a hydrogen atom from β-carbon to metal .

Question 17

Describe the mechanistic step of oxidative addition in metal hydride synthesis.

Answer 17

Oxidative addition involves the metal centre inserting into a bond (e.g., H-H or C-H), increasing the oxidation state of the metal (by 2) and adding two ligands (like H) to the metal centre (increasing e- count by 2).