Ligands that tune the metal

Science · Chemistry · ages 20-21

Name ______________________ Date ____________

Which description defines hapticity rather than denticity?
- The number of separate donor atoms in a chelating ligand
- The total electrons donated by a ligand in the ionic convention
- The number of contiguous ligand atoms attached to one metal
In the sigma-donation part of metal-CO bonding, which way does the electron pair move?
- From a CO sigma donor orbital into a metal acceptor orbital
- From a metal d orbital into a CO pi-star orbital
- From a CO pi-star donor orbital into a metal acceptor orbital
Greater metal-to-CO back-donation tends to strengthen the internal C-O bond and raise its stretching wavenumber.
Circle one: True False
An eta3 allyl anion binds through three contiguous carbons. What is its donation in ionic electron counting?
- Three electrons, equating donation with hapticity
- Four electrons, using the ionic allyl-anion count
- Six electrons, assigning two to each attached carbon
For matched terminal-CO bands with unchanged metal, ligand set and geometry, reduction shifts 2080 to 1970 cm^-1. What does the shift support?
- Less back-donation with a stronger C-O bond
- Greater back-donation with a weaker internal C-O bond
- Greater back-donation with a stronger internal C-O bond
A phosphine substituent is replaced. Which properties should be compared separately?
- Electronic donation and steric crowding as separate properties
- Electronic donation alone; steric bulk stays fixed
- Steric bulk alone; electronic donation cannot change
A student assigns 5 electrons to eta5 Cp- in ionic counting because five carbons attach. What contribution should replace that count?
- Two electrons: treat Cp- as a single lone-pair donor
- Ten electrons: assign a pair to each coordinated carbon
- Six electrons: count Cp- as an ionic six-electron donor
One complex has a lower bridging-CO band than another complex's terminal-CO band. Can this alone rank their metal electron densities?
- Yes; lower bands rank electron density across CO binding modes
- No; differing CO binding modes confound the electron-density comparison
- Yes; bridging and terminal CO respond identically to metal electron density

Answer key

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Ligands that tune the metal W1-mt_JXYSQDYlJ--s1

The number of contiguous ligand atoms attached to one metal · Hapticity describes contiguous attached atoms in a ligand's coordinated system. Denticity describes separate donor sites.
From a CO sigma donor orbital into a metal acceptor orbital · CO supplies a pair from a sigma orbital to the metal. Metal-to-CO donation into pi-star is the separate back-donation component.
False · Back-donation populates CO pi-star antibonding orbitals. It weakens C-O and tends to lower the stretching wavenumber.
Four electrons, using the ionic allyl-anion count · Eta3 is the attached-atom count. The allyl anion is a 4-electron donor in ionic counting.
Greater back-donation with a weaker internal C-O bond · The band decreases by 110 cm^-1. Under the stated controls, that supports stronger back-donation and weaker C-O bonding.
Electronic donation and steric crowding as separate properties · Substituents can affect donation and crowding. The two effects are separate and need not change together.
Six electrons: count Cp- as an ionic six-electron donor · The visible ionic-donation table gives eta5 Cp- as a 6-electron donor. Five attached atoms do not mean five electrons in ionic counting.
No; differing CO binding modes confound the electron-density comparison · Changing terminal to bridging coordination changes the comparison. A frequency difference alone cannot isolate metal electron density.