Lecture 30: Carbonyl Complexes

Read section 23.4 and 23.2-23.3 (partial) from your textbook.

Review: Carbon Monoxide

  1. Properties

  2. Lewis Structure

  3. Molecular Orbital Diagram

    We discussed the molecular orbital diagram of CO in lecture 11.



    CO and metal complexes form pi bonds by the overlap of group orbitals derived from CO pi antibonding orbitals and meta d orbitals.
      Below is an MO diagram of a d6 metal carbonyl complex.





Bonding and Backbonding between Metals and CO

  1. Sigma Donor

  2. Pi Acceptor

  3. Resonance Structures


Other Simple Pi Acceptor Ligands

All of these ligands have filled orbitals with sigma symmetry and empty orbitals with pi symmetry.

  1. CS

  2. NO

  3. NS

  4. N2

  5. O2

  6. PF3

  7. CH2=CH2 (and other alkenes)

  8. HCCH (and other alkynes)
  9. CH3NC (and other isonitriles)


Types of complexes

Carbonyl complexes almost always have an 18 electron count and the metals are in low oxidation states. The carbonyl ligand is small, so complexes with up to 6 carbonyl ligands are very stable.

  1. Homoleptic carbonyl complexes
      When the metal has an even valence electron count, it can make neutral, homoleptic carbonyl complex.
        Examples: Mo(CO)6, Ru(CO)5, Ni(CO)4


      When the metal complex has an odd number of valence electrons, it can make a metal-metal bond to form 18 electron carbonyl complex.
        Examples: (CO)5Tc-Tc(CO)5 (shown below), (CO)4Co-Co(CO)4




      When the metal has an odd valence electron count, it can make cationic or anionic, homoleptic carbonyl complexes.
        Examples: [Mn(CO)6]+, [Co(CO)4]-


  2. Carbonyl clusters

      Transition metals form 2 electron bonds (1 electron from each metal) readily. These bonds are much stronger for 2nd and 3rd row transition metals than for 1st row metals.

      Carbonyl ligands frequently bridge between 2 or 3 metals. Bridging carbonyls are more common in first row transition metals (because the metals are smaller and the M-M bonds are weaker) than in second and third row metals. Compare Fe3(CO)12 and Os3(CO)12 below.



  3. Cyclopentadienyl complexes

      There are many transition metal complexes that have both cyclopentadienyl (Cp) and carbonyl ligands.

      Cp is a 5 electron donor for electron counting purposes and is a -1 ligand for the oxidation state formalism.

      Examples: CpMn(CO)3, CpIr(CO)2, [CpFe(CO)2]2


Spectroscopy

Infrared Absorption Spectroscopy

IR spectroscopy is a type of absorption spectroscopy in which infrared radiation is absorbed by molecules and these molecules are promoted into higher energy vibrational states.

The amount of energy required for a molecular vibration is related to bond strength through the force constant.

In IR spectroscopy, we measure absorption peaks by wavenumbers (cm-1). This is the reciprocal of wavelength and is proportional to energy.


Characteristic Bands for Functional Groups



    Information in the IR spectrum of molecules is frequently used by chemists to identify the molecules.

    Certain vibrations between 4000 and 1500 cm-1 are characteristic of functional groups. Most of the vibrational modes below 1500 cm-1 for organic molecules are due to bending and other deformation modes and are more difficult to identify.

    You should know the approximate positions of the common functional group vibrations listed in the chart to the left from your organic chemistry class.

The C-O stretching vibration of metal carbonyl complexes can tells us about the mode of binding and about the electron density at the metal.



  • 13C NMR Spectroscopy We can get information on the number and symmetry of CO ligands by examining the 13C NMR spectrum of carbonyl complexes. The 13C isotope has a low natural abundance but has a spin of 1/2, like a proton.