Course 201N 1 st Semester Inorganic Chemistry Instructor: Jitendra K. Bera

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1 andout-8 ourse 201N 1 st Semester Inorganic hemistry Instructor: Jitendra K. Bera ontents 3. Organometallic hemistry yclopentadienyl, Alkyl and Alkene

2 yclopentadienyl p The cyclopentadienyl ligand is one of the most common and popular ligands in organometallic chemistry. It is an anionic ligand that normally coordinates in an η 5 mode as a 6e- donor, but it can adopt η 3 - and η 1 -coordination modes. The number of electrons donated is 6, 4 and 2 respectively. Brief istory of Ferrocene: 1901 Synthesis of K 5 5 from K and iller, Tebboth & Tremaine Sythesis of Fe( 5 5 ) 2 from the reaction of 5 6 with freshly reduced Fe at 300º 1951 Kealy & Pauson gbr + Fel 3 p 2 Fe + + 3gBrl They were trying to make fulvalene! They proposed that they had made: Fe 1952 E. O. Fischer proposes a Double-cone structure X-ray structural data Diamagnetism hemical behavior

3 1952 Geoffrey Wilkinson & Robert Woodward: Sandwich Structure IR spectroscopy Diamagnetism Dipole moment = 0 Woodward noted that the p rings were susceptible towards electrophillic substitutions, similar to the aromatic behavior of benzene. Thus the common name: ferrocene 1973 Fischer & Wilkinson receive the Nobel Prize in hemistry for their discovery of ferrocene, which played a key role in opening up the new area of organometallic chemistry. Some Properties of etallocenes omplex olor mp/º Stability V(5 5 ) 2 purple 167 very air-sensitive, paramagnetic r(55)2 scarlet 173 very air-sensitive n(55)2 brown 173 air-sensitive and easily hydrolyzed, interesting high-spin to low-spin interconversion Fe( 5 5 ) 2 orange 173 air-stable, 18e- complex, can be oxidized to blue-green [Fe( 5 5 ) 2 ] +, a good oxidizing agent o( 5 5 ) 2 purple-black 174 air-sensitive, paramagnetic 19ecomplex, is oxidized readily to the airstable 18e- yellow [o( 5 5 ) 2 ] + Ni( 5 5 ) 2 green e- complex, unstable Structural Features The parallel sandwich structures have the following structural features: Distances (Å) - p p - - p p Fe [Fe] Ru o [o] Ni Note the various trends in the bond distances. The changes in the neutral Fe, o, Ni metallocenes are a direct result of going from 18e- (Fe) to 19e- (o) to 20e- (Ni) counts.

4 The extra electrons for the o and Ni complexes are going into -p antibonding orbitals, which are delocalized and progressively weaken the -p bonding, leading to the increase in bond distances. This in spite of the fact that the metal s covalent radius is decreasing as one goes from Fe Ni (effective atomic number contraction effect). Alkyl Alkyls are typically very strong anionic σ-donors, second only to hydrides. They have virtually no π-acceptor ability. β-ydride Elimination One of the most common empty orbital side reactions of alkyls is called the β-hydride elimination reaction: The main driving force for β-hydride elimination is the formation of α carbon a stronger - bond and the generation of an alkene ligand that reduces the unsaturation of β carbon the metal complex. The reverse reaction, however, also can β hydrogen occur and is called a migratory insertion. This is very important in transition metal reaction chemistry and catalysis, as we will discuss this later. Note that in order to have a β-hydride elimination one UST have an empty orbital on the metal cisoidal (next) to the alkyl ligand. One also must have β-hydrogens present on the alkyl. In order to prepare stable -alkyl complexes one, therefore, often needs to stay away from alkyls with β-hydrogens (or avoid metals with empty coordination sites). Some common ligands used to avoid β-hydride elimination reactions are shown below.

5 3 e Si e e e e e methyl neopentyl benzyl trimethylsilylmethyl Alkenes Alkenes act as neutral 2e- donors (per = double bond). Due to the presence of empty π* antibonding orbitals, there is the possibility of π-backbonding: Dewar-hatt- Duncanson bonding model (1953) σ-donation via the filled alkene π-system π-back donation via the empty alkene π -system Alkenes are typically relatively weakly coordinating ligands. They are also extremely important substrates for catalytic reactions. The strongest alkene-metal bonds occur with third row metals (as with almost all ligands) and when one can get more π-backbonding to occur. The amount of π-backbonding depends strongly on how electron-rich the metal center is and whether or not there are electron-withdrawing groups on the alkene to make it a better acceptor ligand. l Pt l Pt(2+) = = 1.37Å Zeiss's Salt l Pt Pt(0) = = 1.43Å N N N N Pt Pt(+2) -- = 1.49Å metallocyclopropane In extreme cases, as shown above to the right, if the metal is electron-rich enough and if there are electron-withdrawing groups on the alkene (like the N s), one can actually get a formal oxidation of the metal via the transfer of 2e- to the alkene to form a dianionic

6 metallocyclopropane ligand that is now coordinated via two anionic alkyl σ-bonds (thus the assignment of Pt(+2)). Another interesting comparison is shown to the F = = 1.40Å right where we have two different alkenes F Rh- = 2.02Å coordinating to the same metal center. The electron-withdrawing fluorine groups on the F F F 2 =F 2 alkene makes it a better π-acceptor ligand. This weakens the = bond, but strengthens the alkene-metal bond. Another series of structures is shown below for butadiene, Fe(η )(O) 3, and p 2 Zr(η ): Rh = = 1.35Å Rh- = 2.16Å 1.45Å 1.46Å 1.46Å 1.40Å 1.36Å O Fe O O Zr 1.45Å In this series one can see that the combination of π-backdonation from the Fe and σ-donation from the alkenes to the Fe weaken and lengthen the = bond. In the Zr complex, however, we see an interesting reversal where the single bond across the back of the butadiene shortens quite a bit. What is happening here is that the Zr is in a very low oxidation state (+2, but it really wants to be +4) and is, therefore, extremely electron-rich. The Zr transfers two electrons to the butadiene via the π-backdonation and generates a metallocyclopentene resonance structure, shown schematically to the right. Zr