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Lewis Bases 1 Lewis Base Ligands Non-carbon donor ligands that have one or more lone pairs of ethat can be donated into empty orbitals on the metal center. Although phosphine ligands (PR3) and hydrides (H) are formally Lewis Base ligands, their importance in organometallic chemistry is such that we will treat them in separate chapters. Halide Donors 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 The halides are anionic Fluoride is generally NOT a good ligand except for very high oxidation state metal centers. It is donors that generally only too electronegative to donate much of its donate 2e- to a metal center. electron density to a “normal” metal center. Due to their relatively high electronegativity they are not especially good -donor ligands. Although they can theoretically act as -donor ligands, once again, the higher electronegativity limits them to simple 2e- donor ligands. Lewis Bases 2 One possible exception to the 2e- donor “rule” when they are coordinating to a single metal center is for iodide. It is the least electronegative of the common halides (not counting astatine) and is the best donor group. This is some evidence that iodide is a good enough donor and has enough orbital extension to act as a 4e- and -donor in some cases. Common Misconception: Since we treat the halides as anionic halide ligands their relatively high electronegativity does NOT make them electron-withdrawing ligands as thought of in organic chemistry. In organic chemistry the halogens are treated as neutral ligands and as such drain electron density from whatever they are attached to. But in inorganic and organometallic chemistry they are anionic and are perfectly happy with that charge. What their electronegativity does in organometallic chemistry is to NOT make the halides particularly good donor ligands. As one moves down the periodic table from F to I, we do see steadily increasing donor ability as the electronegativity drops. When halides act as bridging ligands (relatively common) they donate 2e- to each metal center that they are coordinated to. Lewis Bases 3 Oxygen Donors R O O Alkoxides 2 or 4e- donor Terminal Bridging Triply bridging Carboxylates 2 or 4e- donor 1-terminal 2-terminal Bridging H R R O M O R O O Ethers 2e- donor Typically weak donor O C THF tetrahydrofuran Oxide 4 or 6e- donor Terminal Bridging Triply bridging Quad bridging R O O 2 M M R O O 2 -chelating 4e- O 1 2e- mode M m-bridging R O M Acetoacetonates (acac) 2 or 4e- donor R O H Unusual C-bonded acac M O R Lewis Bases 4 Sulfur Ligands 2e- or 4e- Terminal 4e- Bridging 6e- Triply bridging R S Easily oxidized to R-S-S-R Thiolates Mercaptides Thiolates are powerful bridging ligands, particularly for lowoxidation state metal centers. The lower electronegativity relative to oxygen means that thiolates are also better donors. S 2 4e- Terminal 4e- Bridging 6e- Triply bridging 8e- Quad bridging Sulfide M S M S Relatively rare 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 Lewis Bases 5 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. Nitrogenase, shown to the right,i requires a steady source of electrons. The reaction requires the addition of 8 electrons for each nitrogen molecule that is split into two NH3 molecules and one “wasted” H2. The Fe protein (green) uses the breakage of ATP to pump these electrons into the MoFe protein (blue-purple). Two molecules of ATP are consumed for each electron transferred. The active catalytic site in the enzyme is believed to be this unusual Fe/Mo/S cluster – the central atom was initially believed to be a nitride (N3), see Science, 2002, 297, 1696-1700, but most researchers now favor a C4, carbide, based on XPS and better X-ray structures. i This figure is freely available from the protein databank. http://pdb101.rcsb.org/motm/26 Lewis Bases 6 Nitrogen Ligands In general, alkylated amines are not particularly good ligands. This is mainly due to the relatively short N-C N bond distances and the stereoelectronic problems R R generated from this. R Chelating amines have less steric problems and are better ligands for transition metal centers. Primary and secondary amines, however, are susceptible to oxidations: 2+ H2 N 2+ H N - 4e- M M M=Ru N H2 + 4H+ N H Di-imine Complex Tertiary amines, on the other hand, are quite stable and not easily oxidized. Me Me N Me Me N chelate effect helps coordinate metals more strongly TMEDA = tetramethylethylenediamine Perhaps the most famous neutral nitrogen donor ligand is bipyridine or bipyridyl, almost universally abbreviated bipy. Phenanthroline is a more rigid version of bipy that forms even stronger metal chelates. N N Bipy = Bipyridine N N Phen = phenanthroline Lewis Bases 7 The most famous bipy complex is [Ru(bipy)3]2+, which has an extremely rich photoredox chemistry. There are probably over 1000 N 2+ papers concerning [Ru(bipy)3] . N 2+ N Ru N N N Bipy (and its related systems) can stabilize multiple oxidation states on metal complexes: [Cr(bipy)3]3+ [Cr(bipy)3]1 Inorganic Amides N Strong Base & Nucelophile Terminal (2 or 4e-) donor 4e- Bridging donor 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. Typical formation reaction: MXn + nLiNR2 M(NR2)n + nLiX Amides that act as 2e- donors have pyramidal geometry (sp3-like) with a free lone pair, while amides that donate 4e- have a trigonal planar (sp2-like) geometry. There are, of course, in-between structural cases that indicate intermediate bonding situations. M N M R N R R R 2e- donor pyramidal geometry 4e- donor trigonal planar Lewis Bases 8 Alkyl-Imido (nitrene) Ligand R R N Imido (nitrene) 4e- or 6e- Terminal 4e- Bridging 6e- Triply bridging R N N M M Bent (nucleophillic nitrogen) Linear (electrophillic nitrogen) In the bent mode the imido ligand is formally only a 4e- donor, while in the linear mode it donates 6e- to the metal center. As with the other ligands we have seen that can donate variable numbers of electrons to the metal center, you initially count it as a 4e- donor and then boost it up to 6e- if you need to increase the electron count on the metal to get up to 16 or 18 e-. Some examples: As with amides, you can have intermediate cases where the bending of the alkyl group is in-between linear and bent. It is difficult to predict when the imido ligand will act as a 4e- or 6e- donor. So long as you give me a reasonable electron count I will not be picky about whether you use 4e- or 6e-. If you use 6e- you should realize that the imido and alkyl group should be drawn linear. Lewis Bases 9 Tris(pyrazolyl)borate Ligand H R R B N N N N N N R Anionic 6e- donor coordination via the three lower nitrogen centers Moderate donor Steric effects adjusted via R groups R M 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-. It is considerably bulkier (much bulkier if one uses large bottom R groups) and, due to the higher nitrogen electronegativity, it is not as good a donor relative to Cp. When drawing the ligand, you can abbreviate it as follows: R B N N N M Most inorganic and organometallic chemists will understand what you are talking about with this simplified drawing. Lewis Bases 10 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