Common Ligand Types - University of Victoria

Common Ligand Types

X type ligands: halides F-, Cl-, Br-, I- -
XM

formally anionic: 'halo'

terminal reduced basicity relative Cl
to ‘free’ halide
Cl
Cl W Cl

Cl

Cl

NH3 2+

bridging (μ) H3N NH3 NH3
H3N Cl Cu NH3

Cu Cl NH3

NH3

‘pseudo-halogens’: many other monodentate 1- ligands

egs. CN- ‘cyano’ or cyanide anion (terminal
through C but can also bridge using N)

SCN- ‘thiocyano’ or thiocyanate anion
OH- ‘hydroxo’ or hydroxide ion,
can also bridge

Oxygen donors

water (OH2) alcohols (ROH) and ethers (R2O) are most important

aqua hydroxo oxo

H2O HO- O2-

alcohol alkoxo
ROH RO-

ether
R2O

S equivalents

alkyl / arylthiol alkyl / arylthiolato sulphido
thioether or

alkyl / arylsulphido

charged species far more likely to bridge:

R MO MOM
O
MM terminal bridging

R = H -hydroxo oxo

R = alkyl or aryl -alkoxo or aryloxo

pKa of water drops sharply on coordination to a Lewis acid:
‘free’ water pKw = 14 but [Fe(OH2)6]3+ has a pKa of ca. 2
dn number? d5 hexaaquairon(III)

Increased acidity of coordinated water can have important
consequences on solution chemistry:

Cr(NO3)3 xs H2O [Cr(H2O)6]3+ 3NO3-

+ xs NH3 + xs NH3
[Cr(NH3)6]3+

+ +[Cr(H2O)5(OH)]2+ NH4+ 3 NO3-

- H2O

H 5+

O

(H2O)5Cr Cr(H2O)5

carboxylates, RCO2- R
terminal or -
acetato (R = Me) R

R O O O OR
O OO
O Fe Cr
O R O Cr
O O
O
terminal R O

bridging

R

chelating oxygen donors

neutral: polyethers including crowns and acyclic types

anionic: di- or higher carboxylates (eg. oxalate)
-diketonates (eg. acac)

OO OO O O O
OO dme M M M

12-crown-4 O O O

Ox acac

Nitrogen and phosphorus donors

ammine amido imido nitrido
H3N H2N- HN2- N3-

amino (10) RHN- RN2-
RNH2 R2N-

amino (20)
R2NH

amino (30)
R3N

P equivalents

phosphino phosphido phospheno
or

phosphinideno

anionic versions (amido and imido) are generally good at bridging
amides and phosphides are X- type ligands
pKa drop on coordination is even more dramatic here:

NH3 (pKa ~ 30-33) M←NH3 (pKa ~ 19-22)

Chelating amines and phosphines:

ethylenediamine (en) H2N NH2
diethylenetriamine (dien)
en

N
H

NH2 H2N
dien

bis(diphenylphosphino)ethane (dppe or diphos); related members
of this family (dmpe, dppm); tripodal versions such as
tris(diphenylphosphinomethyl)ethane (triphos) and chiral variants
such as BINAP:

R2P PR2 Me2P PMe2

Ph2P PPhP2 Ph2

dppe (R = Ph) dmpm triphos
dmpe (R = Me)

PPh2 S-BINAP
PPh2

sp2 N donors: N
neutral:
pyridine (py) is the prototype N
N
chelates common here too:
py bipy

anionic:
pyrazolyls (pz)

tris(pyrazolyl)borates -pz N N
sp N donors: MM

[RBpz3]- R

B
N3

N
M

nitriles such as acetonitrile (CH3CN)

dinitrogen (N2)

Carbon monoxide (CO)

Not just a pretty donor!
donates through C end (HOMO is more localized on C)
BUT also
acceptor into empty * levels (LUMO):

empty (dz2) filled (HOMO)
CO
M

filled (dxz) empty (LUMO)
CO
M

Can bridge through carbon (most common) but bridges through C
and O are also known (isocarbonyls):

O O MCOM
isocarbonyl
C C -(C,O)-CO
MM M MM
-CO
-CO