CHAPTER 13 CHAPTER 13. Generated by Foxit PDF Creator Foxit Software For evaluation only.

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Howard White
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1 Generated by Foxit PDF reator Foxit Software Pd- AND f-blk RGANETALLIS PAn organometallic compound must contain P a carbon-metal bond. PBook, p. 459, gives list of common ligands P An over-view of organometallics No. of Electrons gand omplex 1 alkyl R [( 3 ) 3 Ptl] aryl 6 5 ( 6 5 ) 3 r[( 2 5 ) 2 ] 3 2 carbonyl : r() 6 alkene R 2 =R 2 Pt( 2 4 )l 2 3 allyl 2 -= 2 Ni( 3 5 ) 2 4 butadiene 2 =-= 2 Fe() 3 ( 4 6 ) 5 - cyclopentadienyl 5 5 Fe( 5 5 ) 2 6 benzene 6 6 r( 6 6 ) 2 Post of the ligands that donate an even number of electrons contain unsaturated groups. The pi component of ethylene is equivalent to the electron pair on :N 3. PBroadly speaking, there are two kinds of bonding. 1) A single carbon atom is directly attached to the metal: - 2 -= 2 and P2) pi donation: 2 2 P Effective atomic number (EAN) is often used to rationalize bonding. The modern expression of the EAN rule is the 18 electron rule. P We need a method to account for the electrons from the metal atom and those from the ligand. P An example: Fe() 3 ( 4 6 ). With neutral ligands, Fe is formally in a zero oxidation state and so has 8 valence electrons: 4s 2 3d 6. P 3 's donate 2 and 4 6 (butadiene) donates 4. So = 18. P The basis for the 18 electron rule lies in the fact that the ns, np, and (n-1)d electrons are similar in energy.

of Electrons gand omplex 1 alkyl R [( 3 ) 3 Ptl] aryl 6 5 ( 6 5 ) 3 r[( 2 5 ) 2 ] 3 2 carbonyl : r() 6 alkene R 2 =R 2 Pt( 2 4 )l 2 3 allyl 2 -= 2 Ni( 3 5 ) 2 4 butadiene 2 =-= 2 Fe() 3 ( 4 6 ) 5 -

2 Generated by Foxit PDF reator Foxit Software PA couple more examples: r() 6 : r has 6 electrons: 4s 1 3d 5 Each donates two: 2 x 6 = 12 total = 18 n() 5 : n has 7 electrons: 4s 1 3d 5 Each donates two: 2 x 5 = 10 total = 17 But n() 5 actually n 2 () 10 2 n = 14 electrons; 10 = 20 electrons 1 n-n = 2 electrons total = = 36/2 = 18 P d-blk ARBNYLS. Probably the most important organometallic compounds. The molecule has filled 3σ and vacant 2π orbitals. P The sigma orbital is an electron donor, though not a very good one. P The pi orbitals (the LUs)are antibonding orbitals, and thus, of the right symmetry to overlap with d- orbitals on metals. P Backbonding - note that this enables the to bind on metals in low valence states and "bleed" the electron density away from the metal. may form one bond, or may bridge two, or even three, metals

Probably the most important organometallic compounds. The molecule has filled 3σ and vacant 2π orbitals. P The sigma orbital is an electron donor, though not a very good one.

3 Generated by Foxit PDF reator Foxit Software PRelated pi-acid ligands. P N - and N particularly important examples. PNote PF 3 - the d-orbitals on P, once thought to be the acceptors, play little role. PIt is the P-F antibonding sigma orbital formed from the P 3p orbitals. Pany polynuclear carbonyls. Pften prepared by thermal or photochemical decomposition of the simple carbonyls 2 Fe() 5 --hν--> Fe 2 () 9 + PSTRUTURE. Generally adhere to the 18-electron rule. P The () 6 are octahedral, the () 5 are trigonal bipyramid (care -the monomeric ones are!), and () 4 are tetrahedral. Notable exception is V() 5, which esr shows to be monomeric down to liquid nitrogen temps and, below that dimerizes and then follows the 18-electron rule. As noted before, the 18- electron rule helps us figure out that n() 5 is actually n 2 () 10 Pne of the main tools for determining the structure of the carbonyl compounds is vibrational spectroscopy. P Free has its IR stretch at 2143 cm -1. The coordination of should reduce the (triple) bond order, because electrons from the metal have been accepted into the antibonding orbitals. That is what happens - we see, for Fe() 5, = stretch of 2116, 2030, and 1989 cm -1. Three since there are three posssible symmetries for the vibrations of the set of five ligands. Two are IR active and three are Raman active. Infrared Raman Totally symmetric band - absent in IR, present in Raman

Pften prepared by thermal or photochemical decomposition of the simple carbonyls 2 Fe() 5 --hν--> Fe 2 () 9 + PSTRUTURE. Generally adhere to the 18-electron rule.

4 Generated by Foxit PDF reator Foxit Software PIn polynuclear molecules, we have basically two types of 's - terminal and bridinging. PBridging 's have even lower frequencies - there are two metals donating electrons into the antibonding orbitals on one molecule. P For Fe 2 (η 5 - p) 2 () 4, we get two terminal stretches in the area of 2000 cm -1, and one peak (from the bridging ligands) at 1800cm -1. Terminal Bridging Fe Fe P Two types organometallic compounds are formed: ionic and covalently bonded. PGroup IA metal alkyls provide examples of both extremes. Pthium alkyls are soluble in hydrocarbons and other nonpolar solvents indicative of covalent character. They react violently in air and water, often igniting spontaneously. Pther Group IA metal alkyls are appreciably more ionic than are the lithium compounds. P They are not particularly soluble in hydrocarbon solvents, but they are still extremely reactive toward air and moisture. P These ionic organometallic compounds can be prepared either by treatment of dimethylmercury with the appropriate alkali metal or by the reaction of an alkyl halide with an alkali metal: P( 3 ) 2 g + 2 K --> 2[ 3 ] - K + + g P 2 5 l + 2 K --> [ 2 5 ] - K + + Kl

P For Fe 2 (η 5 - p) 2 () 4, we get two terminal stretches in the area of 2000 cm -1, and one peak (from the bridging ligands) at 1800cm -1.

5 Generated by Foxit PDF reator Foxit Software PStructures typical of ionic compounds; for example [ 3 ] - K + crystallizes in the nickelarsenide lattice. P thium alkyls have different structures than the other metal alkyls; Pdue to their greater covalent character, they form tetrameric or octahedral clusters, for example, ( 3 ) 4 and ( 25 ) 6. The structure of the former is a square: PThese small clusters, called oligomers, are attributed to multicenter, two-electron bonds l > ( 3 ) l P Groups IIA and IIIA metal aklyls also form clusters or polymers. P onomeric precursors for Groups IA-IIIA elements - P eg R, R 2 Be, and R 3 Al - electron-deficient so P achieve more stable electronic configurations by forming multicenter bonds. Such as: PPreparation of these types of main-group alkyls include: l > ( 3 ) l 3( 2 5 ) 2 g > ( 2 5 ) g x Bel 2 + 2x (n-bu) --> [(n-bu) 2 Be] x + 2x l 110 o x[( 3 ) 2 g] + x Be > [( 3 ) 2 Be] x + x g 3 3 Al 3 3 Al 3 3 3( 3 ) 2 g + 2 Al ---> ( 3 ) 6 Al g 2 Al > ( 2 5 ) 6 Al 2

The structure of the former is a square: 3 3 3 PThese small clusters, called oligomers, are attributed to multicenter, two-electron bonds.

6 Generated by Foxit PDF reator Foxit Software P ay also use lithium alkyls as starting material to make more complex compounds: P p 2 Fe + n-bu --> p( 5 5 )Fe + n P Sodium cyclopentadienide can used also. P Prepared by reacting cyclopentadiene with sodium metal dispersed in tetrahydrofuran. yclopentadiene not stable as a monomer The dimer must be cracked thermally immediately prior to use: P heat > P Group IA organometallics used to synthesize ketones: P 2 ( 3 ) > 4 ( 3 ) P thium alkyls used as stereospecific polymerization catalysts to form 1,4-cispoly(isoprene). P Aluminum alkyls important in the Ziegler- Natta alkene polmerization. P And, don t forget the Grignard reagents, RgX P Na ---> 2 Na + p P Synthesis of Group IIIA and Group IVA organometallics: P Gal gbr --> ( 2 5 ) 3 Ga + 3 gbrl P Tl --> Tlp+ 2 P Sil gl --> ( 3 ) 2 Sil gl 2 P l + 4Na/Pb --> ( 2 5 ) 4 Pb + 3Pb + 4Nal P The compounds all highly covalent, soluble in organic solvents, and fairly volatile.

yclopentadiene not stable as a monomer The 10 12 dimer must be cracked thermally immediately prior to use: P heat 10 12 -----------> 2 5 6 P Group IA organometallics used to synthesize ketones: P 2 (

18 electron rule : How to count electrons

18 electron rule : How to count electrons The rule states that thermodynamically stable transition metal organometallic compounds are formed when the sum of the metal d electrons and the electrons conventionally