Transcript Slide 1
Lewis Base Ligands -- Halides Increasing polarizability X Strongest nucleophile for low oxidation state metals centers F , Cl , Br , I Primary Halides M X 2e- terminal X X M M 4e- m-bridging M M M 6e- m3 -bridging 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 electronegativity 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 Cubanes SR Fe S Fe S RS S Fe 2- SR Fe SR S Fe S RS 2- RS S Ferredoxins R S 3- Fe S Fe SR RS 2Fe(+2) 2Fe(+3) R S S R S Fe Fe S SR S R Nitrogenase The 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 S Fe Fe R S O S RS Fe Fe Mo Fe Fe S S N S S Fe O N(his) S O Nitrogen Donors N R R R 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. 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. N N Bipy = Bipyridine N N Phen = phenanthroline Inorganic Amides Strong Base & Nucelophile Terminal (2 or 4e-) donor 4e- Bridging donor N R R The lone pairs in an amide are about 2eV higher in energy than in OR-. This makes an amide a considerably stronger donor. M N M R N R R R 2e- donor pyramidal geometry 4e- donor trigonal planar Alkyl-Imido (nitrene) Ligand R R R N - 4e- or 6e- Terminal 4e- Bridging 6e- Triply bridging Imido (nitrene) N N M M Bent (nucleophillic nitrogen) Linear (electrophillic nitrogen) Me Ph 3 Ta Os O O N N N O R2N NR 2 Cl NR2 Re P Cl Ph3P Cl As one moves to the right-hand side of the periodic table, one tends to get less M-L multiple bonding Tris(pyrazolyl)borate Ligand H R R R B N N N N N N M • • • • • Anionic 6e- donor coordination via three lower nitrogen centers Moderate donor Steric effects adjusted via R groups R 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 N M N A Few Biologically Important N-Ligands N N N N N N pyrimidine imidazolate N H N purine 3 2 4 1 NH N N HN 5 8 7 6 Porphine (metalloporphyrin when bound to a metal) Protein S S Protein N S Protein N N Fe N HOOC N Fe N N COOH Cytochrome-C HOOC N HOOC Fe-protoporphyrin IX