Study Notes on Metal-PI Complexes and Metal Carbonyls: Structure, Synthesis, and Bonding

Overview of Metal-$\Pi$ Complexes and Metal Carbonyls

• Definition: Metal carbonyls are coordination compounds consisting of transition metals bonded to carbon monoxide ($CO$) ligands.
• General Bonding Concept: While most coordination compounds involve the donation of a ligand's electron pair to a metal center, metal carbonyls involve a more complex interaction known as $\pi$-backbonding (or $\pi$-backdonation).
• $\pi$-Backbonding Mechanism: This occurs when a ligand has both filled atomic orbitals (donor orbitals) and empty orbitals (acceptor orbitals) with appropriate symmetry and energy. The ligand donates a $\sigma$ electron pair to the metal, and the metal center simultaneously donates electron density from its filled d-orbitals back into the ligand's empty $\pi^*$ orbitals.
• Common Ligands for Backbonding: Includes $CO$, $NO$, $PR_3$, and alkene-alkyne type ligands.
• Key Examples: $Ni(CO)_4$ and Zeise's salt are classic examples where $\pi$-backbonding is prominent.

Significance and Toxicity of Metal Carbonyls

• Applications:
  • Used extensively in synthetic organic chemistry.
  • Vital in homogeneous catalysis, specifically the process of hydroformylation.
  • Mond Process: Utilizes nickel carbonyl to produce ultra-pure nickel.
  • Precursors: Act as starting materials for synthesizing various other organometallic compounds.
• Biological Toxicity: Metal carbonyls are highly toxic via inhalation, skin contact, or ingestion.
  • Mechanism: They bind to the iron center in hemoglobin to form carboxyhemoglobin.
  • Effect: This interaction is stronger than that of dioxygen ($O_2$), thereby inhibiting the binding and transport of oxygen in the blood.

Classification of Metal Carbonyls

• Based on Metal Centers:
  • Mononuclear Carbonyls: Contain a single metal atom or ion (e.g., $Fe(CO)_5$, $Ni(CO)_4$).
  • Polynuclear Carbonyls: Contain multiple metal centers.
    • Homonuclear: Multiple centers of the same metal (e.g., $Fe_2(CO)_9$).
    • Heteronuclear: Centers of different metals (e.g., $MnRe(CO)_{10}$).
• Based on Bonding Profile:
  • Non-bridging (Terminal) Carbonyls: Carbonyl groups attached to only one metal. These may occur with or without metal-metal ($M-M$) bonds.
    • $Ru(CO)_5$ has no $M-M$ bond and only terminal ligands.
    • $Mn_2(CO)_{10}$ has a metal-metal bond but only terminal carbonyls.
  • Bridging Carbonyls: Carbonyl groups that are shared/bonded to more than one metal center (e.g., $Fe_3(CO)_{12}$).

General Methods of Preparation

• Direct Reaction: Direct combination of carbon monoxide with metal powder.
  • $Ni + 4CO \xrightarrow{25^\circ C, 1\,atm} Ni(CO)_4$
  • $Fe + 5CO \xrightarrow{200^\circ C, 200\,atm} Fe(CO)_5$
  • $2Co + 8CO \xrightarrow{150^\circ C, 35\,atm} Co_2(CO)_8$
• Reduction of Metal Salts: Using reducing agents in the presence of $CO$.
  • With Aluminum: $CrCl_3 + Al + 6CO \xrightarrow{AlCl_3, \text{benzene}} Cr(CO)_6 + AlCl_3$
  • With Sodium: $VCl_3 + 4Na + 6CO \xrightarrow{\text{diglyme}, 100^\circ C} [(diglyme)_2Na][V(CO)_6] + 3NaCl$
  • With Copper: $2CoI_2 + 4Cu + 8CO \xrightarrow{200^\circ C, 200\,atm} Co_2(CO)_8 + 4CuI$
  • With Hydrogen: $2CoCO_3 + 2H_2 + 8CO \xrightarrow{120-200^\circ C, 250-300\,atm} Co_2(CO)_8 + 2H_2O + 2CO_2$
  • $CO$ as Reducing Agent: $Re_2O_7 + 17CO \xrightarrow{250^\circ C, 350\,atm} Re_2(CO)_{10} + 7CO_2$
• Photolysis of Mononuclear Carbonyls: Irradiation of mononuclear complexes can yield binuclear versions.
  • $2Fe(CO)_5 \xrightarrow{h\nu} Fe_2(CO)_9 + CO$
  • Also applicable to $Os(CO)_5$ and $Ru(CO)_5$.
• Ligand Lability from Iron Pentacarbonyl: $Fe(CO)_5$ can serve as a reactant to synthesize other carbonyls due to the labile nature of its ligands.
  • $MoCl_6 + 3Fe(CO)_5 \xrightarrow{110^\circ C, \text{ether}} Mo(CO)_6 + 3FeCl_2 + 9CO$
  • Same process applies for $WCl_6$.
• Metathesis Reaction: Synthesis of mixed-metal clusters.
  • $KCo(CO)_4 + [Ru(CO)_3Cl_2]_2 \rightarrow 2RuCo_2(CO)_{11} + 4KCl$

Structural Analysis of Metal Carbonyls

• Mononuclear Structures:
  • Tetrahedral: $Ni(CO)_4$, $Pd(CO)_4$.
  • Trigonal Bipyramidal: $Fe(CO)_5$, $Ru(CO)_5$, $Os(CO)_5$.
  • Octahedral: $V(CO)_6$, $Cr(CO)_6$, $Mo(CO)_6$, $W(CO)_6$.
• Binuclear Structures:
  • $Co_2(CO)_8$: Exists as two isomers. One has $D_{3d}$ symmetry (one $M-M$ bond, no bridges); the other has $C_{2v}$ symmetry (two bridging $CO$, one $M-M$ bond).
  • $Fe_2(CO)_9$: Has $D_{3h}$ symmetry with three bridging and six terminal $CO$ groups.
  • $M_2(CO)_{10}$ ($M = Mn, Tc, Re$): Features $D_{4d}$ symmetry with a single $M-M$ bond and four $CO$ ligands per metal center.
• Polynuclear Structures:
  • $Ru_3(CO)_{12}$ and $Os_3(CO)_{12}$: $D_{3h}$ symmetry, equilateral triangle of metals with two axial and two equatorial $CO$ ligands per metal.
  • $Fe_3(CO)_{12}$: $C_{2v}$ symmetry due to presence of two bridging $CO$ ligands.
  • $M_4(CO)_{12}$ ($M = Co, Rh$): Tetrahedral core with $C_{3v}$ symmetry (three bridging ligands, nine terminal).
  • $Ir_4(CO)_{12}$: Perfect $T_d$ symmetry with no bridging ligands.
  • Stability Trend: Cluster robustness increases down a group ($Rh_4$, $Ir_4 > Co_4$).
  • Anionic Clusters: $[Re_4(CO)_{16}]^{2-}$ has $D_{2h}$ symmetry and no bridges.
  • Osmium Clusters:
    • $Os_4(CO)_{16}$: Puckered structure analogous to cyclobutane.
    • $Os_4(CO)_{15}$: Planar butterfly-like geometry.
    • $Os_4(CO)_{14}$: Irregular tetrahedral skeleton.
  • Hexanuclear Clusters: $M_6(CO)_{16}$ ($M = Rh, Co$) features an octahedral core with four triply bridging faces and twelve terminal ligands.

Bonding Theories for Metal Carbonyls

Valence Bond Theory (VBT)

• $CO$ Molecule Bonding: Carbon and oxygen are $sp$-hybridized.
  • Configuration for Carbon: $1s^2 (sp_x)^2 (sp_x)^1 2p_y^1 2p_z^0$
  • Configuration for Oxygen: $1s^2 (sp_x)^2 (sp_x)^1 2p_y^1 2p_z^2$
  • A $\sigma$ bond forms from the overlap of half-filled $sp_x$ orbitals. Two $\pi$ bonds are formed: one from sidewise overlap of half-filled $2p_y$ orbitals, and one dative bond from Oxygen's filled $2p_z$ into Carbon's empty $2p_z$.
• Metal Interaction: The metal provides empty hybrid orbitals (e.g., $d^2sp^3$ for $Cr(CO)_6$) to accept $\sigma$-donation from the Carbon lone pair.
• Preferred Bonding Site: Bonding always occurs via the carbon atom because the carbon lone pair is higher in energy than the oxygen lone pair.

Molecular Orbital Theory (MOT)

Three generations of MO diagrams describe $CO$:

• First Generation: Assumes simple interaction between C and O.
  • Configuration: \sigma_{2s}^2 \sigma^*_{2s}^2 \sigma_{2pz}^2 \pi_{2px}^2 \pi_{2py}^2
  • Anomalies: Incorrectly predicts that ionization to $CO^+$ would decrease bond order (by removing a bonding electron), whereas bond order actually increases. Also fails to explain why $CO$ binds linearly rather than perpendicularly.
• Second Generation (Coulson's Model): Suggests carbon and oxygen undergo hybridization before creating molecular orbitals.
  • HOMO is now a non-bonding hybrid lone pair ($sp_x$) on Carbon.
  • Explains linear bonding and the availability of $\pi^*$ LUMO for backbonding.
  • Remaining Paradox: Does not explain why removing an electron from a non-bonding HOMO ($CO \rightarrow CO^+$) increases the bond order.
• Third/Modern Generation (Accepted Model):
  • Accounts for four $\sigma$ and two doubly degenerate $\pi$ molecular orbital sets.
  • The $\sigma_5$ orbital (HOMO) is slightly bonding and centered mostly on Carbon.
  • The Bond Order Solution: Ionization of $CO$ removing an electron from the $\sigma_5$ orbital causes a shift in electron polarization. The partial positive charge on Carbon lowers the energy of its atomic orbitals, aligning them closer to Oxygen's orbitals. This results in enhanced covalent character and stronger interaction, effectively shortening the bond and increasing bond order in $CO^+$ despite the loss of a bonding electron.