Organometallic Ligands: Carbonyls, Phosphines and Hapticity
Organometallic ligands affect metal bonding and reactivity. CO stretching frequencies can compare back-donation in otherwise similar complexes. Phosphine substituents affect electronic and steric properties. Hapticity describes how many contiguous ligand atoms attach to a metal, not how many electrons they donate.
What a learner can do afterwards
- 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
1 · Read
You have counted ligand donations. Now distinguish binding descriptions. Denticity counts separate donor atoms: water binds through one oxygen, while a chelating ligand binds one metal through several donor atoms and forms a ring. Hapticity counts contiguous atoms in a coordinated pi system. An eta2 alkene uses two adjacent carbons; eta3 allyl uses three; eta5 cyclopentadienyl uses five. Neither count alone gives the electron donation.
CO bonds through carbon. It donates an electron pair from a sigma orbital to the metal, and it can accept metal d-electron density into its pi-star antibonding orbitals. This back-donation weakens the internal C-O bond and tends to lower its stretching wavenumber. The two directions of donation contribute to metal-CO bonding.
Compare corresponding terminal-CO bands in complexes with the same metal, ligand set and geometry, measured under comparable conditions. If reduction shifts a band from 2120 to 2010 cm^-1, that decrease supports greater back-donation and a weaker C-O bond. An added electron can increase metal-to-CO donation. Changing the ligand set or binding mode may change the band for other reasons.
State the ligand charge and counting convention. An eta2 alkene donates 2 electrons in both methods. An eta3 allyl anion donates 4 in ionic counting or 3 in neutral counting. An eta5 cyclopentadienyl anion, Cp-, donates 6 ionically or 5 neutrally. Hapticity gives 2, 3 or 5 attached atoms; it is not a rule of one donated electron per atom.
A phosphine, PR3, is a 2-electron ligand in either counting method. Its substituents affect electronic donation and the space it occupies near the metal. Electron-releasing substituents often increase donation; larger substituents can crowd a reaction site. Steric and electronic effects are separate properties. A particular substitution need not change both by the same amount or in the same direction.
An IR comparison must compare like with like. Terminal and bridging CO ligands can have different stretching bands, and several CO groups can give coupled modes. A lower band in an unrelated complex is not enough to rank metal electron density. Check the metal, ligand set, geometry, CO binding mode, assigned vibration and measurement conditions before drawing that conclusion.
Back-donation weakens C-O. Compare matched IR bands. State ligand charge and counting method; hapticity is an atom count. Phosphines have separate steric and electronic effects.
Original lesson, questions and flashcards on carbonyl bonding, phosphines and hapticity, written for this learning collection.
Original reading on ligand binding modes, electron donation and controlled carbonyl-band comparisons, written for this learning collection.
2 · Watch
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