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Transcript
5.03, Inorganic Chemistry
Prof. Daniel G. Nocera
Lecture 11 Apr 11: Hydride and Dihydrogen Complexes
Hydride and dihydrogen are both 2e– donors, H– (1s2) and H2 (σ1s2)
Hydride complexes are synthesized by:
(1) Replace halide with hydride using hydride transfer reagents:
(2) Heterolytic cleavage of a dihydrogen complex:
(3) Oxidative-addition of hydrogen to a metal complex:
There are some general features of H2 oxidative-addition:
• cis addition
• 16e– complexes or less add H2 (since 2e–s are added to the metal complex)
• bimolecular rate law (rate = k [IrL2Cl(CO)] [H2])
• ∆H‡ = 11 kcal/mol (little H–H stretch in the transition state recall that
BDE(H2) = 104 kcal/mol), and a ∆S‡ = 21 eu
• rate decreases along the series X– = I– > Br– > Cl– (100 ; 14 : 0.9)
• little isotope effect, kH / kD = 1.09
1
For the oxidative-addition reaction, there are two possibilities for the transition
state:
1
an H2 intermediate
2
a three-center transition state
Both reaction pathways are viable for oxidative-addition (and the reverse reaction,
reductive-elimination).
For some metal complexes, the “arrested” addition product can be isolated—the
dihydrogen complex is obtained as a stable species that can be put in a bottle.
Kubas first did this in 1984 with the following reaction:
2
Several observables identify this as an authentic dihydrogen complex vs. a
dihydride:
• d(H—H) = 0.84 Å (as measured from neutron diffraction). This distance is
near the bond distance of free H2, d(H—H) = 0.7414 Å.
• a symmetric H2 vibration is observed, ν(H—H) = 2,690 cm–1, as compared
to ν(H—H) = 4,300 cm–1 in free H2.
• JHD = 33 Hz (vs 43 Hz in HD). For a dihydride complex JHD ~ few Hz.
There is a delicate balance between a dihydrogen and dihydride complex:
From dynamic NMR measurements at 300 °C: k1 = 63 s–1 (∆G‡ = 15.2 kcal/mol)
k–1 = 12.4 s–1 (∆G‡ = 16.0 kcal/mol)
Therefore the equilibrium constant is Keq = 5.0.
Ancillary ligands play an important role in determining the energetic balance
between the dihydrogen and dihydride complexes. For instance, replacement of the
3 COs in the Mo version of Kubas’ original complex with PR3s furnishes the
dihydride as the energetically favored complex
The delicate balance between dihydrogen and dihydride complex can be understood
with orbital comparisons to ethylene complexes. An ethylene complex is balanced
3
energetically between a metallocyclopropane (all σ complex) and the olefin π
complex.
σ-bond: filled 2e– π bond
donates into empty dz2 / pz
π-bond: empty π* of ethylene
accepts 2e–s from metal dπ
orbital
A similar situation arises for H2 complexes. The frontier ligand orbitals in this case
are σ(H2) and σ*(H2).
Note: if metal too electron rich, π-back
donation into H2 σ* orbital will lead to H–H
cleavage and hence dihydride formation
4
More quantitatively,
M–H2 *
*(b2)
M–H2 *
dz 2– y 2
eg
dz 2
dyz
dxz
M–H2
t2g
dxy
(a1)
M–H2
M
M
H
H
H
H
z
M
y
x
Thus the M–H2 π interaction is important for stabilizing the dihydrogen complex, but
too much π-backbonding will break the H–H bond.
5
Reactivity
(1)
(2)
Isomerization
Hydrogenation of Alkenes and Alkynes
6
(3)
Alkene Insertion
Broken
M–H
P(C=C)
Formed
~60 kcal/mol
M–C
35-40 kcal/mol
64 kcal/mol
C–H
98 kcal/mol
~124 kcal/mol
133-138 kcal mol
∆H = –9 to –14 kcal/mol (but there is an unfavorable entropic term). Thus
the overall process is near thermoneutral and typically forward and reverse
reactions are run with equal facility
(4)
Brønsted acid/base chemistry
7