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Alkenes like CO are poor 2-electron donors.

Lecture 32 Structure and reactivity of organometallic compounds 1) Bonding in transition metal olefin complexes. Alkenes like CO are poor 2-electron donors.

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Alkenes like CO are poor 2-electron donors.

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  1. Lecture 32 Structure and reactivity of organometallic compounds 1) Bonding in transition metal olefin complexes Alkenes like CO are poor2-electron donors. • But in contrast to carbonyl, regular alkenes are also poorp-acceptors and thus poor ligands. They form only weak bonds with transition metals. Weak olefin complexes are important intermediates in catalytic olefin transformations. The strongest metal – olefin complexes known are formed by 5d-elements: K[PtCl3(C2H4)] (Zeise salt, first organometallic compound; discovered in 1827), Pt(C2H4)3, IrCl(C2H4)4 etc. M-olefin bonding. Electrons of the olefinic p-bond which is the HOMO in the ethylene ligand (-10.2 eV) are donated to an empty orbital of the metal to form the M-olefin s-bond. • In addition, electrons from one of the filled d-orbitals of the metal are p-backdonated to the LUMO (5.0 eV) of the ethylene (M-olefin p-backbonding). Strong p-backbonding leads to cyclopropane-type structures. • If there are strong electron-withdrawing groups on alkene (see examples below) and if the metal is very electron-rich (Pt0, TiII), the degree of the p-backbonding increases in such an extent that the metal can be oxidized by the alkene to form cyclopropane-type products.

  2. 2) MO diagram of ethylene • CH2=CH2 (D2h) (E, C2, C2, C2, sv, sv’, sv”)

  3. 3) Consequences of the M-olefin p-backbonding • The consequences of a strong M-olefin p-backbonding are: 1) the olefinic C=C bond becomes longer; 2) three atoms attached to the olefinic carbon are no more in one plane with it; 3) the olefin C=C bond stretching frequency decreases. • The p-backbonding prevents from the free rotation about M-olefin sigma-bond

  4. 4) Alkyne complexes. MO diagram of acetylene • HC≡CH (D∞h)

  5. 5) Consequences of the M-alkyne p-backbonding • p-Backbonding leads to distortion of the linear C-C≡C-C unit, elongation of the C≡C bond (compare with l(C≡C)=1.20 Å in the free acetylene): • As a result, strained cycloalkynes and arynes can be stabilized by coordination to a metal:

  6. 6) W(PhC≡CPh)3(CO), 20-electron complex or not? • C3v (E, 2C3, 3sv) p-bonding to CO is not shown:

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