Water Ligands

Water Ligands (H₂O)

Electron Contribution: 2 electrons (neutral ligand).

Introduction:

Water (H₂O) is one of the simplest yet most versatile ligands in organometallic chemistry. Despite being a weak-field ligand, it plays crucial roles in stabilizing metal centers, facilitating proton transfer, and participating in catalytic cycles—particularly in aqueous organometallic chemistry and biological systems.

Key Features:

• Weak field ligand, common in hydrated complexes.
• σ-donor, weak π-acceptor: good at stabilizing medium to high oxidation state metals. 
• Easily displaced, making it useful in catalytic cycles.
• Protic Nature: can participate in hydrogen bonding and O-H activation.
• Example: [Fe(H₂O)₆]³⁺.

Reactivity:

Proton Transfer/Acidity:

Water ligands can dissociate protons, generting hydroxo (OH^-) ligands.

[M(H₂O)]⁺ ⇔ [M(OH)] + H⁺

Hydroxo ligands can go on to react with CO₂ to form carbonic acid:

Another water molecule can bind to the unoccupied coordination site to continue the cycle; present in carbonic anhydrase to maintain blood pH.

Ligand Exchange: Water is often a placeholder ligand, being displaced by stronger donors (e.g. CO, phosphines, olefines):

[M(H₂O)_n] + CO → [M(H₂O)_n-1CO] + H₂O

O-H Bond Activation: Electron rich metals can cleave water oxidatively: 

[M] + H₂O → [M-OH] + H⁺ + e^-

Utility:

Aqua complexes are used in a number of reactions to catalyze the functionalization, oxidation, and reduction of different substrates. 

Functionalization:

• Water can participate in hydrofunctionalization reactions by anti-Markovnikov addition of alkenes.
  • RCH=CH₂ + H₂O → RCH₂CH₂OH
  • Catalysts: [Pt(H₂O)(PPh₃)₂]²⁺- used in hydration of ethene into ethanol.

Proposed Image: Octahedral model of [Fe(H₂O)₆]³⁺ showing water ligands.