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Common Types

- X type ligands: F-, Cl-, Br-, I- X M formally anionic: 'halo'

Cl

Cl terminal reduced basicity relative Cl W Cl to ‘free’ 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 anion (terminal through C but can also bridge using N)

SCN- ‘thiocyano’ or anion OH- ‘hydroxo’ or , can also bridge 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. ) -diketonates (eg. acac)

O O O O O M M M O O O O O O O

12-crown-4 dme Ox acac 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 and :

H2N NH2 (en) en

N H (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: (py) is the prototype N

N chelates common here too: N py bipy

anionic: pyrazolyls (pz)

-pz NN tris(pyrazolyl) MM R

B N 3

N [RBpz ]- 3 M sp N donors:

nitriles such as (CH3CN)

dinitrogen (N2) (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