How Can CN-, P(CH3)3, and SCN- Act as Both σ-Donors and π-Acceptors?

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ReidMerrill
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Homework Statement


Describe, using sketches, how the following ligands can act as both σ-donors and π-acceptors:
CN-
P(CH3)3
SCN-

Homework Equations

The Attempt at a Solution



I suspect it has something to do with these ligands having lone pair electrons AND open π* orbitals.
How correct is this and if it is wrong then what is the explanation?
 
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Discussion

ReidMerrill said:
I suspect it has something to do with these ligands having lone pair electrons AND open π* orbitals.
How correct is this and if it is wrong then what is the explanation?
This is basically correct. Look up "pi-backbonding" for a good explanation. Bonus fact: Electron backdonation into antibonding orbitals turns out to be really important in catalysis where bond breaking is important.