---
title: "Organometallic Ligands: Carbonyls, Phosphines and Hapticity"
description: "The ligands of organometallic chemistry are chosen for what they do to the metal, not just for binding. Carbon monoxide reports on electron density through its stretching frequency, phosphines are tun"
canonical: https://lightmysky.com/learn/science/organometallic-ligands-carbonyls-phosphines-and-hapticity-mt_JXYSQDYlJ-
source: https://lightmysky.com/learn/science/organometallic-ligands-carbonyls-phosphines-and-hapticity-mt_JXYSQDYlJ-.md
retrieved: 2026-09-12
---

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# Organometallic Ligands: Carbonyls, Phosphines and Hapticity

The ligands of organometallic chemistry are chosen for what they do to the metal, not just for binding. Carbon monoxide reports on electron density through its stretching frequency, phosphines are tuned by their substituents, and carbon chains can bind through several atoms at once.

Subject: Science · Area: Chemistry · Ages 20 to 21
Page: https://lightmysky.com/learn/science/organometallic-ligands-carbonyls-phosphines-and-hapticity-mt_JXYSQDYlJ-

## Ready when they can

- Explains back-donation into the carbonyl antibonding orbital and predicts the effect on its stretching frequency
- Uses a measured carbonyl frequency to compare electron density at two metal centres
- Assigns hapticity for alkene, allyl and cyclopentadienyl ligands and counts their electron contribution
- Explains how changing a phosphine's substituents changes both its donor strength and its size

## Lesson: Ligands that tune the metal

A coordination compound is a team: a metal centre plus ligands, where each ligand lends the metal an electron pair in a coordinate bond. A monodentate ligand holds on with one donor atom, like water through its oxygen. A polydentate ligand wraps round with several donor atoms, forming a chelate that grips like a claw.

Carbon monoxide does more than just bind. It donates sigma density to the metal and then accepts metal density back into its empty pi orbitals. That return gift weakens the CO bond itself, so the stretching frequency drops. The richer the metal centre, the more back donation and the lower the frequency: compare a neutral carbonyl with its anion and the anion absorbs lower.

**Example.** Carbon chains can bind through several neighbouring atoms at once, and the Greek prefix names the touch count: that is hapticity. An alkene touches through two atoms, an allyl through three, and a cyclopentadienyl ring through five. Count the touching atoms to tally the electrons the ligand contributes.

**Tip.** Phosphines are the tunable ligands of the set. Changing their substituents shifts both their donor strength and their bulk at the metal. So when comparing two centres, read the carbonyl frequency for electron richness and check the phosphine substituents for donor power and size.

**Recap.** CO reports richness through its frequency, hapticity counts touching atoms, and phosphines tune donation and bulk.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Reading Bond Lengths and Force Constants from Spectra](https://lightmysky.com/learn/science/reading-bond-lengths-and-force-constants-from-spectra-mt_5m7J1mFw68)
- [Electron Counting and the Eighteen-Electron Rule](https://lightmysky.com/learn/science/electron-counting-and-the-eighteen-electron-rule-mt_uab5c5pODY)

## Opens up

- [Olefin Metathesis and Ring-Closing Strategy](https://lightmysky.com/learn/science/olefin-metathesis-and-ring-closing-strategy-mt_J9UAxKsgSk)
- [Elementary Steps of Catalysis: Oxidative Addition and Insertion](https://lightmysky.com/learn/science/elementary-steps-of-catalysis-oxidative-addition-and-insertion-mt_VVLGGh8QZ_)