Q1(i). Back-donation and carbonyl stretching

Answer in brief

The amine donor dien increases electron density at Mo, strengthening Mo→CO π back-donation and weakening the remaining C–O bonds.

Key points

• Dien is diethylenetriamine, H₂NCH₂CH₂NHCH₂CH₂NH₂, a neutral tridentate nitrogen donor.
• Both compounds contain formal Mo(0), d⁶; in Mo(dien)(CO)₃, three CO ligands are replaced by the three donor sites of dien.
• Dien is a strong σ donor with little π-acceptor ability; the more electron-rich Mo donates more strongly into the π* orbitals of the remaining CO.
• Greater π* occupancy lowers C–O bond order and force constant k, so the stretching wavenumber ν̃ = (1/2πc)√(k/μ) falls.
• The decrease from about 2000 to 1760 cm⁻¹ reflects enhanced back-bonding, not a change of Mo oxidation state.