Back-bonding

• CO acts as p-acceptor ligand

• dp-p* back bonding to unfilled antibonding p* orbitals

• decreases triple bond strength (1.15 Å)

• thus n decreases (1850-2125 cm-1)

• free CO 1.128 Å , 2143 cm-1

anti-

bonding p*

filled metal

d orbitals

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