Metal-Metal Bonding in Engel-Brewer Intermetallics: “Anomalous” Charge Transfer in ZrPt₃

Ab initio generalized valence bond, configuration interaction, and multiconfiguration self-consistent field calculations have been performed to examine the properties of the early-late transition metal bimetallic cluster ZrPt₃, as a representative model of Engel-Brewer compounds. Such intermetallic compounds are known to be extremely thermally stable in the bulk form. We find the atomization energy of the ZrPt₃ cluster at its bulk geometry to be at least 101.1 kcal/mol, which supports the Engel-Brewer suggestion that an alloy of Zr and Pt should be particularly stable. However, we find that the charge transfer occurs in the opposite direction from that assumed by the Engel-Brewer theory; namely, the Zr atom is predicted to donate approximately one electron to the three Pt atoms. The high thermal stability of these compounds is attributed to a combination of localized, highly polar, sd-sd bonds between Zr and Pt that enhance the normal metallic (sp-sp) bonding present in homometallic Pt clusters. In order to better understand intermetallic metal-metal bonding and charge transfer, calculations for low-lying states of ZrPt dimer have also been carried out. Bond energies, vibrational frequencies, equilibrium geometries, and charge distributions are predicted.