TT-Acceptor Ligands Notes

TT-Acceptor Ligands

Features of π-acceptor ligands

• Possess vacant low-lying orbitals (e.g., π* or d-orbitals).

• Examples include CO, NO, PR3, normal & cyclo-alkenes, acetylenes, isonitriles, etc., with low-valent transition metals (TMs).

• For these ligands (L), visualizing bonding solely as L→M donation is inadequate.

• Interaction leads to metal-ligand multiple bonds.

MO Diagram of CO

The molecular orbital (MO) diagram of CO shows the atomic orbitals of C and O combining to form sigma (σ) and pi (π) bonding and antibonding MOs:

• Atomic orbitals: 2s and 2p for both C and O.

• Molecular orbitals: σ(3σ), σ(5σ), π(1π), π(2π), σ(4σ), σ*(6σ)

The diagram illustrates the energy levels and interactions of these orbitals in the CO molecule.

Bonding in Metal Carbonyls

(a) Formation of the C→M σ bond:

• Involves an unshared pair of electrons on the carbon atom donating to the metal.

(b) Formation of the M→C π bond (back-bonding):

• Metal dπ electrons are donated into the π* antibonding orbitals of CO.

• This back-bonding strengthens the M-C bond and weakens the C-O bond.

Probing the CO Environment

Two main physical methods support the multiple bond nature of M▬C bonds:

(i) Bond lengths

(ii) Vibrational spectra (IR spectroscopy)

IR spectroscopy is based on Hooke’s law, which relates the vibrational frequency to the bond strength and masses of the atoms.

IR Signatures of CO Ligands

Coordination modes of CO ligands and their corresponding IR frequencies (cm⁻¹):

• Free CO: 2143

• Terminal M-CO: 2120 - 1850

• Doubly bridging M₂-CO: 1850 - 1750

• Triply bridging M₃-CO: 1730 - 1620

• μ₂-bridging: M₂-CO

• μ₃-bridging: M₃-CO

Typical IR Spectrum of a Carbonyl Complex

An example IR spectrum of a carbonyl complex, such as Fe₂(CO)₉, shows characteristic peaks corresponding to the CO stretching frequencies. The number and position of these peaks provide information about the structure and symmetry of the complex.

Points to Note

• Back-bonding lowers the C▬O bond order and consequently the νCO (C-O stretching frequency).

• Examples of νCO values (cm⁻¹) for different carbonyl complexes:

  • $[Ni(CO)_4]$: 2060

  • $[Mn(CO)_6]^+$: 2090

  • $[Co(CO)_4]^-$: 1890

  • $[Cr(CO)_6]$: 2000

  • $[Fe(CO)_4]^{2-}$: 1790

  • $[V(CO)_6]^-$: 1860

• Molecular symmetry is directly proportional to the number of IR peaks.

Application of IR to Metal-dicarbonyls

The intensity of IR peaks can provide information about isomerism in non-linear carbonyl complexes.

• Let V = dipole vector for the CO oscillator.

• θ = angle between two CO ligands.

Dicarbonyl Dipole Vectors

• Symmetric stretch: The resultant dipole vector (Vsym) is the vector sum of the individual CO dipole vectors.

• Asymmetric stretch: The resultant dipole vector (Vasym) is the vector difference of the individual CO dipole vectors.

Resultant Dipole Vectors

• Two situations for the resultant dipole vector:

  • $V_{sym}$

  • $V_{asym}$

• Intensities are proportional to the square of the dipole vector:

  • $I<em>{sym} ∝ (V</em>{sym})^2 = (2v\cos(\frac{θ}{2}))^2$

  • $I<em>{asym} ∝ (V</em>{asym})^2 = (2v\sin(\frac{θ}{2}))^2$

  • $\frac{I<em>{sym}}{I</em>{asym}} = \cot^2(\frac{θ}{2})$

Phosphines

• As electronegative R groups are added, the phosphorus atom (and the metal to which it's bound) become more electron-poor.

• Phosphines (:PR3) & Phosphites [:P(OR)3).

• CO equivalents, but use empty 3d orbitals as acceptors.

• The nature of R in PR3 is important due to steric and electronic effects.

Factors affecting back-donation

(i) The identity of the donor atom (N, P).

(ii) The χPauling of the groups attached to it.

Write short notes on the influence as probed by νCO in IR spectroscopy.

(iii) The overall charge on the complex ion.

Special Types of Organometallics

Organometallic compounds of transition metals have unusual structures and practical applications in organic synthesis and industrial catalysis.

• Define an organometallic compound.

• Define the hapticity (η) of a ligand.

(A) η²-Alkene Complexes

• Zeise’s salt (1827): Unconventional bond.

• Structural consequences.

• Free C-C bond length = 134 pm.

Bonding in Olefin Complexes

• Another example of synergic bonding!

• Called the Dewar-Chatt-Duncanson (DCD) bonding model:

  • The ligand donates π electrons to a metal orbital of σ symmetry directed to the center of the ligand system.

  • The metal in turn back-donates electron density into a ligand π* orbital.

Simplified Version of Olefin Bonding

Illustrates the donation and back-donation between the metal and the olefin ligand.

• Overall effect on C-C double bond:

Signatures of Synergism in Olefin Complexes

Related to those in Metal-Carbonyl Complexes:

(i) C─C bond distances

(ii) C─C stretching frequencies (νC-C) in IR

(iii) Chemical Reactivity (Organic Chemistry 1st-level)

Coordination v/s νC═C (IR spec.)

Compound ν(C═C)/cm-1

• $[Rh(Cp)(C<em>2H</em>4)_2]$: 1493

• $[PtCl<em>2(C</em>2H<em>4)]</em>2$: 1506

• $[Mn(Cp)(C<em>2H</em>4)(CO)_2]$: 1508

• $K[PtCl<em>3(C</em>2H<em>4)] · H</em>2O$: 1516

• $[PdCl<em>2(C</em>2H<em>4)]</em>2$: 1525

• Free $H<em>2C=CH</em>2$ has a ν(C═C) of 1623 cm-1

Equilibria involving olefin complexes

• Have, Olefin Relative Keq

  • $H<em>2C=CH</em>2$: 1

  • $(NC)HC=CH(CN)$: 1 500

  • $(NC)<em>2C=C(CN)</em>2$: 140 000

• Comment on these data & extend to C-C distances.

Confidence-building questions

Rationalize the following observations:

(a) On going from $Fe(CO)<em>5$ to $Fe(CO)</em>3(PPh<em>3)</em>2$, absorptions in the IR spectrum at 2025 and 2000 cm-1 are replaced by bands at 1944, 1886 and 1881 cm-1 . [See p.471 M&T, 2nd .]

(b) On forming $IrBr(CO){η^2-C<em>2(CN)</em>4}(PPh<em>3)</em>2$, the unique C-C bond in $C<em>2(CN)</em>4$ lengthens from 135 to 151 pm.

(c) The Tolman cone angles of $PPh<em>3$ and $P(p-MeC</em>6H<em>4)</em>3$ are 145°, but that of $P(o-MeC<em>6H</em>4)_3$ is 194°.