1. Dative ligands with more than one donor atom s Metal complexes
Textbook H: Chapter 2.8
Textbook A: Chapter 8.7.4

2. Metal dihydrogen complexes
Discovered in 1984 by Kubas: M(CO)3(PR3)2(H2) (M = Mo, W; R = Cy, i-Pr)
Bonding
dH-H = 0.84 Å (0.74 Å in free H2); dW-H = 1.75 Å
nHH = 2690 cm-1; nHD = 2360 cm-1
dHH = (24 Hz wide), dHD = (8 Hz wide, 1:1:1 triplet,
JHD = 33 Hz, JHD (HD gas) = 43 Hz)
Good p acceptors (CO, NO)
are effective in stabilizing
the H2 ligand.
Reference: Kubas, G. J. et al. J. Am. Chem. Soc. 1984, 106, 451

3. Agostic interactions Characterization
Agostic: Greek for “to hold on to oneself”; term coined by Brookhart.
It refers to a C-H/Si-H bond, which interacts with a metal.
B: first agostic complex,
Cotton, 1974
C is more stable than either
the alkyl or the alkene-
hydride complexes.
Characterization
1H NMR: peak shifted from that of a normal aryl or alkane towards that of a hydride ligand
JCH is typically around Hz versus the 125 Hz of a normal sp3 C atom
Usually, fluxional, exchange between agostic and terminal Hs
IR: sometimes, a reduced nCH can be observed
Neutron diffraction: dM-H in agostic complexes is from 1.85 to 2.4 Å
B: Mo-H, 2.1 Ǻ; IR, 2704 and 2664 cm-1; 1H NMR, -3.8 ppm

4. Alkane coordination Possible coordination modes: X-ray Reed 1997 X-ray
Meyer 2003
h2-H,C

5. Published in: James A. Calladine; Simon B. Duckett; Michael W
Published in: James A. Calladine; Simon B. Duckett; Michael W. George; Steven L. Matthews; Robin N. Perutz; Olga Torres; Khuong Q. Vuong; J. Am. Chem. Soc.  2011, 133 (7), pp 2303–2310
DOI: /ja110451k
Copyright © 2011 American Chemical Society

6. Metal Hydrocarbyls and Related Ligands
Textbook H: Chapter 3.2.1, 3.8
Textbook A: Chapter 8

7. Stability and reactivity
First metal alkyls: 1848, Frankland, EtZnI and ZnEt2.
Alkyl compounds of main group elements (Al, Mg, Si, Sn, Pb) followed the discovery of zinc alkyls.
Similar alkyl compounds of TMs were considered unstable until 1960s.
Main group alkyl
DHf (kcal/mol)
BDE (kcal/mol)
TM alkyl
CMe4
SiMe4
GeMe4
SnMe4
PbMe4
-40
-59
-17 -5
+32 86 75 60 52 36
Ti(CH2tBu)4
Zr(CH2tBu)4
Hf(CH2tBu)4
TaMe5
WMe6 47 64 62 38

8. Binding modes

9. Bond strengths for classical s-ligands
Classical s-bonding ligands (X = H, CH3, Cl) form strong M-X bonds.
Bond dissociation energies (BDEs) of organometallic compounds are more difficult to determine than for organic compounds. Useful in predicting reaction outcome.
There is a good correlation between M-X BDE and H-X BDE, except LnM-H is usually stronger than LnM-CH3 for middle to late TM. Explanation: destabilizing orbital interactions.
References: Martinho Simöes, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629
Ziegler, T. Pure & Appl. Chem. 1991, 63, 873 (DFT)

10. Decomposition temp (°C)
General trends
M-C bond enthalpy increases within a TM group. In main group alkyls, the M-C bond enthalpy decreases down in a group.
M-C: kcal/mol
Similar to main group alkyl bond strength.
Comparable to the strength of a C-I bond.
The instability of TM alkyls is kinetic not thermodynamic.
One strategy to isolate TM alkyls is to block available orbitals by using coordinating ligands (bipy, phosphines).
Decomposition temp (°C)
TiMe4
TiEt4
~ -50
not observed
PbMe4
PbEt4
~ 200 (b.p. 110)
~ 100

11. Kinetic instability: b-H elimination
Modes of blocking b-H elimination
No b-hydrogens
The alkyl is oriented so that the beta position cannot access the metal center (steric bulk or rigidity).
The alkyl would give an unstable alkene as the product.

12. a (or g)-H elimination
a-hydride elimination
g-hydride elimination

13. Other decomposition modes
Reductive elimination: isolable alkyl hydrides are rare because the reaction is kinetically facile and thermodynamically favorable. The reverse reaction: C-H activation
Homolytic cleavage: rare

14. Empirical relative stabilities
1-norbornyl > benzyl > trimethylsilyl > neopentyl > Ph ~ Me >> Et (1° R) > 2 °, 3 ° R
Fluoroalkyl > alkyl (i.e. -CnF2n+1 > -CnH2n+1 for late TMs) CF bonds are very strong ( kcal/mol vs kcal/mol for alkyl C-H).
Chelating (metallacycles) > nonchelating (acyclic)
3rd row > 2nd row > 1st row transition metals

15. Metal alkyls: bonding and characterization
1H and 13C NMR
d for the C and H atoms a to the metal are shifted to high field vs. those in the parent alkane.
Coupling of the 13C and 1H nuclei of the alkyl to the metal with spin I = ½: 103Rh (100% abundance); 195Pt (34%); 183W (14%); 199Hg (17%); 187Os (1.6%) or to phosphines (if present).
X-ray: M-C bonds are 1.9 – 2.2 Å
Reactivity
M-R + Br2 gives M-Br
M-R + HgCl2 gives R-HgCl

16. Synthesis: nucleophilic attack on the metal
Very useful and rather general method
Common reagents are RLi, RMgX (or R2Mg), ZnR2, AlR3, BR3, and PbR4.

17. Metal hydrides: importance and characterization
1931, Hieber reports H2Fe(CO)4
, Cp2ReH, PtHCl(PR3)2, K2[ReH9]
M-H bonds can undergo insertion reactions
with unsaturated substrates
Characterization
1H NMR
d: +25 to -60 ppm (usual: -5 to -15 ppm)
Coupling
With the metal (if it has I = 1/2)
With cis and trans phosphines: stereochemistry determination
With each other (if inequivalent, J = 1 – 10 Hz)
IR: n(M-H) 1500 – 2000 cm-1, not very useful (weak intensities)
Neutron diffraction (vs. X-ray diffraction): large crystals are needed (1 mm3 vs mm3)

18. Metal hydrides: synthesis from a ligand
From alkyl ligands:
a-hydride elimination: Ziegler-Natta polymerization, carbene formation
b-hydride elimination: stability of metal-alkyl complexes
From other ligands

19. Silyl complexes: M-SiR3 (R = alkyl, aryl, OR)
First complex: CpFe(CO)2(SiMe3), Wilkinson 1956
Trimethylsilyl (TMS) complexes are more numerous than t-Bu complexes (rare)
b-elimination inhibited due to instability of the Si=C
bonds
Sterically less congested because M-Si is longer
than M-C
M-SiR3 bonds are stronger than M-C bonds due to
p-interaction between M and SiR3 fragment
Most common synthesis method: oxidative addition of Si-H bonds (in contrast to C-H activation)
Rich chemistry

20. Amide, oxides, and halide ligands
Extra lone pairs present on the heteroatom
Late TM: when 18e, repulsion between lone pairs and filled d orbitals; weakening of the M-heteroatom bond
Early TM: when d0, empty orbitals available for p interaction: strengthening of the M-heteroatom bond