METAL METAL-BONDING IN TRANSITION-METAL CLUSTERS WITH OPEN D-SHELLS - PT3

Ab initio generalized valence bond with configuration interaction calculations have been performed for low-lying electronic states of Pt3. The calculations utilize the relativistic effective core potential developed by Hay and Wadt. Under the constraint of C2n symmetry, seven states derived from three d9s1-like Pt atoms (3A1, B-5(2), 5A1, 5A2, 3B1, 3A2, and B-5(1) are predicted to have geometries slightly distorted from equilateral triangles, while two other states involving one d10-like Pt and two d9s1-like Pt atoms (1A1 and 3B2) retain equilateral triangle geometries without distortion. Only one low-lying electronic state ((5)DELTA(g)) has a linear geometry, arising from all three Pt atoms in d9s1 states. All 10 states are predicted to lie within 6.6 kcal/mol of each other. Other states are found to be at least 10 kcal/mol higher in energy than the ground state (3A1). The calculated high density of electronic states and the presence of a low-lying linear structure provide some insight into the experimental spectra of platinum trimer. The role of s-s bonding versus d-d bonding in the formation of the Pt3 cluster is discussed, with the major conclusion that s-s bonding is dominant, while d-d electronic interactions are far less important. However, d-d electronic coupling has a small contribution to the geometric distortion away from an equilateral triangle. Finally, we predict that the Pt3 ground state is bound by at least 50.3 kcal/mol with respect to three separated 3D Pt atoms.