Ab initio calculations with relativistic effective core potential have been performed to study Pdn-CO clusters with n ranging from 1 to 7. Attention has been focused on the comparison between the results of "Lowest Spin State Principle" and those of "Metallic State Principle". Better agreement between metallic Pd~n-CO clusters and CO/Pd chemisorption systems demonstrates that the "Metallic State Principle" overweighs the "Lowest Spin State Principle". According to the "Metallic State Principle" the bonding mechanism in CO/Pd adsorption system can be described as CO(5σ)→Pd(5sp) σ donation and Pd(5sp)→CO(2π^*) π backdonation.

Key words: CHEMICAL ADSORPTION, AB INITIO CALCULATION, ATOMIC ORBITAL, VARIETY