Common Ligand Types
- X type ligands: halides F-, Cl-, Br-, I- X M formally anionic: 'halo'
Cl
Cl terminal reduced basicity relative Cl W Cl to ‘free’ halide Cl Cl
NH 2+ 3 NH3 NH3 Cl Cu NH bridging (μ) 3 H3N Cu Cl H N 3 NH 3 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 - 2- H2O HO O
alcohol alkoxo ROH RO-
ether R2O
S equivalents
alkyl / arylthiol alkyl / arylthiolato sulphido or thioether alkyl / arylsulphido
charged species far more likely to bridge:
R O M M MO MOM
R = H -hydroxo terminal bridging
R = alkyl or aryl -alkoxo or aryloxo oxo pKa of water drops sharply on coordination to a Lewis acid:
3+ ‘free’ water pKw = 14 but [Fe(OH2)6] has a pKa of ca. 2 dn number? d5 hexaaquairon(III)
Increased acidity of coordinated water can have important consequences on solution chemistry:
xs H O Cr(NO ) 2 [Cr(H O) ]3+ 3NO - 3 3 2 6 3
+ xs NH + xs NH 3 3
3+ [Cr(NH3)6] 2+ + - [Cr(H2O)5(OH)] + NH4 + 3 NO3
- H2O
H 5+ O (H O) Cr 2 5 Cr(H2O)5
- carboxylates, RCO2 R terminal or - R acetato (R = Me) O O R O O O O Cr Cr O Fe O R O O O O O O R R terminal bridging R chelating oxygen donors neutral: polyethers including crowns and acyclic types anionic: di- or higher carboxylates (eg. oxalate) -diketonates (eg. acac)
O O O O O M M M O O O O O O O
12-crown-4 dme Ox acac Nitrogen and phosphorus donors
ammine amido imido nitrido
H N H N- 2- 3- 3 2 HN N
amino (10) RNH RHN- RN2- 2
amino (20)
- R2NH R2N
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:
H2N NH2 ethylenediamine (en) en
N H diethylenetriamine (dien) NH H N 2 2 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:
Me2P PMe2 R2P PR2 PPh Ph2P 2 PPh2 dppe (R = Ph) dmpe (R = Me) dmpm triphos
PPh2 S-BINAP PPh 2 sp2 N donors: neutral: pyridine (py) is the prototype N
N chelates common here too: N py bipy
anionic: pyrazolyls (pz)
-pz NN tris(pyrazolyl)borates MM R
B N 3
N [RBpz ]- 3 M sp N donors:
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)
M C O
filled (dxz) empty (LUMO)
M C O
Can bridge through carbon (most common) but bridges through C and O are also known (isocarbonyls):
O O
C C M C O M M M M M isocarbonyl M
-CO -CO -(C,O)-CO