SCF-Xa-SW calcns. are carried out on Cp2YbC2H2 and Cp2Yb(OC)2, both of the nonrelativistic and relativistic schemes have been tried. Discussing orbital interactions from energy sequences, contour maps and populations, a weak interaction between C2H2 and Yb is shown, which is in agreement with experiments CO interacts with Yb more strongly than C2H2 does, namely, coordinating effect of s ligand is stronger than that of p one. There are some stronger covalent bonds in Cp2Yb(OC)2 than those in Cp2YbC2H2, but both of their bonds are less covalent in character than those in three valence Ln compounds such as Cp2Sm and LnF3. Ln components for bonding orbitals are in the order: d>f>p>s. HOMO are Cpp orbital for nonrelativistic calcns., but Ln 4f one for relativistic treats due to relativistic indirect effect makes Ln4f level rise up, which is consistent with the experimental result that the two valence Ln compounds are unstable and easily oxidized.

Key words: CHEMICAL BONDS, YTTERBIUM COMPLEX, MOLECULAR ORBITAL THEORY, ACETYLENE P, CYCLOPENTANE P, SANDWICH COMPOUNDS