The bonding and the 4f orbital effect of lanthanide elements at different valence state in their compounds have been studied by INDO method. The bonding of lanthanide compounds is affected by many factors, such as valence state, ionic radius, ligand, coordinate no., space configuration etc. The strength of bonds composed of different ligands with lanthanide is distinctly different. The covalence of Ln-L bonds of lanthanide ions at high valence state in their compounds is larger than that at low valence state. Teh covalency at low coordinate no. is larger than that at high coordinate no. Some lanthanide compounds with special configuration, besides s bond, can form pp-dp dative bond with much overlap, which makes the Ln-L bond increase markedly. The effect of 4f orbitals on bonding is far less than that of 5d orbitals. The Ln 4f orbitals at 3 or 2 valence state may be considered to be essentially localized while the contribution of 4f orbitals on bonding in 4 valent Ce compounds increases obviously, up to 1%.

Key words: MOLECULAR ORBIT, CHEMICAL BONDS, LANTHANIDE SERIES COMPOUNDS