Strong chemical bonds in heavy diatomics: PtSi, PtTh and AuTh þ Maria Barysz a,b, * , Pekka Pyykk€ o b a Department of Quantum Chemistry, Institute of Chemistry, Faculty of Chemistry, Nicolaus Copernicus University, Gagarina 7, PL-87 100 Toru n, Poland b Department of Chemistry, University of Helsinki, P.O.B. 55 (A.I. Virtasen aukio 1), FIN-00014, Helsinki, Finland Received 12 November 2002 Abstract Ab initio relativistic calculations are reported for the electronic ground states of the known diatomic species PtSi and PtTh, and the unknown isoelectronic species AuTh þ . Relativistic effects are treated at the quasirelativistic Douglas– Kroll level and electron correlation at the CASPT2 or CCSD(T) level. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction We have recently presented theoretical evidence for triple bonding to gold in the predicted diatomic species AuBC þ [1]. That species was mass-spec- troscopically observed shortly afterwards, and the determined dissociation energy was close to the calculated one [2]. This may be the only known triple bond to gold; a formula like AuBN does not correspond to reality because the two extra elec- trons, compared to AuC þ , occupy an antibonding p  orbital and no ways were found to put them somewhere else in a theoretical search [3]. In AuC þ , the Au 5dp shell suffers a substantial de- localization to the vacant C 2pp shell. This valence isoelectronic series also contains the well-known PtC [4] and the bimetallic PtTh [4,5]. Correlated ab initio calculations for the former [6] and a Dirac– Fock calculation without Th 5f orbitals for the latter [7] exist. Recently the valence isoelectronic PtSi was made [8,9] and its properties suggested multiple bonding, which prompts a theoretical study. We also perform the first theoretical study of PtTh with an adequate basis and with electron correlation included. Furthermore, we determine the expected properties of the isoelectronic AuTh þ , which could have another multiple bond to gold. Putting the issue into a broader context we note that a search of multiple MM bonds among the heavier Group 14 elements M@SiAPb is an active field of study. While these bonds are difficult to make, multiple MM 0 bonds, where M 0 is a transi- tion metal, are easy. In the present series the latter bonds can be studied in their simplest, diatomic form. Chemical Physics Letters 368 (2003) 538–541 www.elsevier.com/locate/cplett * Corresponding author. +48-56-654-2477. E-mail address: teomjb@chem.uni.torun.pl (M. Barysz). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2614(02)01893-6