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.

Oxygen Donors

Sulfur Donors

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

A Few Biologically Important N-LigandsN

N NH

N

N N N N

imidazolate pyrimidine purine

NH

N HN

N

1

2 3

4

5

67

8

Porphine(metalloporphyrin when bound to a metal)

N

N N

N N

N N

NFe

SProtein

S Protein

COOHHOOC

Cytochrome-C

S Protein

HOOCHOOC

Fe

Fe-protoporphyrin IX