Monohapto

Monohapto is a term used in coordination and organometallic chemistry to describe a ligand that binds to a central metal atom through a single atom. It is shorthand for a hapticity of one, often denoted by η^1 in chemical formulas and discussions of bonding modes. The concept of hapticity measures how many contiguous atoms of a ligand are bound to the same metal center.

In practice, many common ligands act as monohapto donors. Examples include halide ions (such as Cl−, Br−, I−), hydride (H−), water (H2O), ammonia (NH3), and various alkyl or aryl groups (for instance CH3−) when they donate through a single atom. Cyanide (CN−) also typically binds through carbon as an η^1 donor. Monohapto ligands are usually described as σ-donor, delivering electron density from one atom to the metal.

Ligands can also exhibit higher hapticity, binding through more than one atom to the metal. For instance, ethylene can behave as η^2, allyl as η^3, cyclopentadienyl as η^5, and benzene as η^6. A single ligand may switch between different hapticities depending on the metal, oxidation state, or coordination geometry, though monohapto binding is common for simple, non-multidentate ligands.

Notation and implications: When describing a complex, η^1 indicates monohapto binding of the ligand, while higher η^n values indicate multi-atom binding. Hapticity is an important concept for understanding geometry, reactivity, and electron counting in coordination compounds.