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Grade: 12th Pass
        54. The stretching frequency for free CO molecule is ~2143 cm-1. In Fe(CO)5 molecule the

stretching frequency for CO becomes
(A) ~2240 cm-1
(B) ~2030 cm-1
(C) remain unchanged
(D) cannot be predicted
7 years ago

Answers : (1)

Sunil Kumar FP
askIITians Faculty
183 Points
							it wiil be lowered
therefore the answer will be (B)2030 cm-1
reason for this is back bonding.
A description of the bonding of π-conjugated ligands to a transition metal which involves asynergicprocess with donation of electrons from the filled π-orbital or lone electron pair orbital of the ligand into an empty orbital of the metal (donor–acceptor bond), together with release (back donation) of electrons from annd orbital of the metal (which is of π-symmetry with respect to the metal–ligand axis) into the empty π*-antibondingorbital of the ligand
The electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of CO (and its analogues). This electron-transfer (i) strengthens the metal-C bond and (ii) weakens the C-O bond. The strengthening of theM-CO bond is reflected in increases of the vibrational frequencies for the M-C bond (often outside of the range for the usual IR spectrophotometers). Furthermore, the M-CO bond length is shortened. The weakening of the C-O bond is indicated by a decrease in the frequency of the νCOband(s) from that for free CO (2143cm−1),
5 years ago
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