what are metal hydrides used for
hydrogen storage
batteries (normally solid state materials)
catalysis (often molecular complexes)
are metal hydrides organometallic complexes
not rllyyy
but their reactivity is similar and they often contain C donor ligands
what are the several coordination modes of metal hydrides (M-H)
terminal (M-H)
u-H : triangle of M M H
u3-H: triangular based pyramid with MMMH
interstitial: Oh type angelo shape with M on each corner and H in the middle
how do we synthesise transition metal hydrides
- protonate a metal carbonyl anion
- hydrogenation
- reaction with a main group hydride source
describe how we synthesise metal hydrides by protonating a metal carbonyl anion
[Mn2(CO)10] + Na/Hg —>
[Mn(CO)5]- + H+ —> [MnH(CO)5]
describe how we make metal hydrides via hydrogenation
[IrCl(CO)(PPh3)2] + H2 (1atm, 25*c) —> [Ir(H)2Cl(CO)(PPh3)2]
this is called oxidative addition
describe how we make metal hydrides by reacting a main group hydroxide source
[FeBr2(CO)4] + NaBH4 —> [Fe(H)2(CO)4]
all MH complexes are called metal hydrides but not all of them have what character
not all of them have hydridic character
aka H- character
if not all metal hydrides have hydridic character,, what else can they be
they can be strong acids
aka they have H+ character
if we look at the periodic table and close into the transition metals,,, which ones have H+ character and which ones have H- character
think of electronegativity
the ones on the LHS are more electropositive and therefore the H will be H- (hydridic character)
the ones on the RHS are more electronegative and therefore the H will be more H+ aka more acidic
this is a rule of thumb!! ligands and other factors also influence it
can some terminal hydrides be acidic
yessss
they can be strong acids
when we think of a metal hydride complex what should we think of
think of a normal complex but add H to the metal
and then lowks remove a ligand
so u have Co with H and 4CO
when we think of a strong acid what does this dissociate into and what are our products
strong acid —> conjugate base ( no H complex) + H+
CO ligands are what type of ligands
they’re rlly good pi acceptor ligands so we get wnxwnionic conjugate base
and it’s stable!!
bc the electron density can be donated to pi*
this makes our acid strong bc the conj base is stable
when we see a conj base what does it normally have
it normally has a - sign on the side of the complex
do ligands influence acidity and if so how
yes! ligands and metal (from before) influence acidity
if it’s a good pi acceptor ligand it can result in a stable conjugate base and bc conj base is stable,, it means our acid is a strong acid aka very acidic
what’s good at characterising if the H is - or + and why
H NMR
bc H- is heavily shielded and therefore will have a very low chem shift,, aka negative ppm
-5 to -10 ppm realistically
or 50 to -50
HNMR OF H wirh no shielding aka a d0 metal
ppm of 7.5
bc there’s no shielding
why is x-ray diffraction not the best at characterising H
bc the intensity of diffraction is proportional to the atomic number,, so it’s difficult to locate
why isn’t neutron diffraction good at characterising H
the intensity of diffraction is independent of atomic number
aka we need a neutron source
what types of reactions are important in catalysis
H transfer reactions are important in catalysis
what H transfer reactions are rlly important in catalysis
- migratory insertion reactions
- outer sphere H transfer
describe a migratory insertion reaction
Cp2Zr(H)Cl + RCH=CH2 —> Cp2ZrCl(Ch2CH2R)
