Q1(j). Why ordinary lanthanoid carbonyls are unstable

Answer in brief

Ordinary lanthanoid ions bind CO weakly: their contracted 4f orbitals provide poor π back-donation and Ln³⁺ favours hard donors.

Key points

• A stable conventional metal carbonyl normally uses synergic CO → metal σ donation and metal → CO π back-donation.
• Lanthanoid 4f orbitals are radially contracted and shielded by 5s and 5p shells; overlap with CO π* is poor.
• The prevalent Ln³⁺ state is strongly electropositive and a hard Lewis acid, favouring oxygen donors such as water and carboxylate over neutral CO.
• Weak back-donation makes ordinary neutral carbonyl binding insufficient for readily isolable classical binary carbonyls.