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 2+ NH3 NH3 NH3 Cl Cu NH bridging (μ) 3 H3N Cu Cl H3N 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 - 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 2 3+ - Cr(NO3)3 [Cr(H2O)6] 3NO3 + xs NH3 + xs NH3 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 O R 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) 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: 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) C O M 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.