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Lewis Base Ligands -- Halides
X
XX X
F , , , Cl Br I
Increasing polarizability
Primary Halides
Strongest nucleophilefor low oxidation statemetals centers
MM
M
MMM
2e- terminal 4e- -bridgingm 6e- -bridgingm3
Common Misconception: The halides are anionic ligands, so they are NOT electron-withdrawing ligands. In organic chemistry the halogens can be considered neutral ligands and do drain electron density from whatever they are attached to. But here they are anionic and are perfectly happy with that charge. Their electronegativity makes the halides “poor” donor ligands. As one moves from F- to I-, the donor ability increases as the electro negativity drops.
Sulfur Donors
Sulfides (and thiolates) are extremely effective bridging ligands and play a critically important role as such in bioinorganic chemistry:
Fe
S
S
S
Fe
Fe
Fe
S
SR
RS
SR
2- FeS
FeSR
R
R
R
S
S
S
S
SR
RS S
S
SS
R
R
R
R
2-
S
Fe
S
Fe
S
Fe
3-Ferredoxins
2Fe(+2)2Fe(+3)
Fe/S Cubanes
NitrogenaseThe nitrogenase enzyme catalyzes the extremely difficult reaction:
N2 + 8H+ + 8e- 2NH3 + H2
The conversion of atmospheric N2 into
NH3 is a marvel of catalysis and
provides the plant with its own fertilizer.
In the Haber catalytic process for
converting N2 + 3H2 into NH3, over 400ºC
and 400 atm of H2/N2 pressure are
required along with a heterogeneous Fe catalyst.
Nitrogenase does this at room temperature and pressure!!
S
Fe
FeFeS
S Fe
S
Fe
S
S
FeS
Fe
S
Mo
S
NRS
N(his)
O
O
O
R
Nitrogen Donors
In general, alkylated amines are not particularly good ligands. This is mainly due to the relatively short N-C bond distances and the stereoelectronic problems generated from this.
NR
RR
N N N N
Bipy = Bipyridine Phen = phenanthroline
Chelating amines have fewer steric problems and are better ligands for transition metal centers. Perhaps the most famous neutral nitrogen donor ligand is bipyridine or bipyridyl, almost universally abbreviated bipy.
Inorganic Amides
N
RR
Strong Base & NucelophileTerm inal (2 or 4e-) donor4e- Bridging donor
The lone pairs in an amide are about 2eV higher in energy than in OR-. This makes an amide a considerably stronger donor.
NM
RR
M NR
R
2e- donorpyram idal geom etry
4e- donortrigonal planar
Alkyl-Imido (nitrene) Ligand
NRIm ido (nitrene)
4e- or 6e- Term inal 4e- Bridging6e- Triply bridging M M
N NR R
Bent(nucleophillic
nitrogen)
Linear(electrophillic
nitrogen)
OsO
OO
N N N
TaNR2
NR2
R2NCl
Cl
Cl
Ph3P
P
Me
RePh3
As one moves to the right-handside of the periodic table, one tendsto get less M-L multiple bonding
H
B
N NN
N NN
R
R R
R
M
• Anionic• 6e- donor• coordination via three lower nitrogen centers• Moderate donor• Steric effects adjusted via R groups
Tris(pyrazolyl)borate Ligand
The tris(pyrazolyl)borate (Tp) ligand has become extremely popular in the last decade and is sometimes called the inorganic Cp ligand. It is anionic and is a 6e- donor when all three nitrogens are coordinated to a metal center, just like Cp-. R
B
N NN
M