FULL PAPER Quadruple chemical bonding in the diatomic anions TcN - , RuC - , RhB - , and PdBe - Demeter Tzeli 1,2 1 Laboratory of Physical Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Athens, Greece 2 Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Athens, Greece Correspondence Demeter Tzeli, Laboratory of Physical Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, Athens 157 84, Greece. Email: tzeli@chem.uoa.gr Funding information National and Kapodistrian University of Athens, Special Accounts for Research Grants, Grant/Award Number: SONFM:17034 Abstract Quadruple bonding is uncommon for main group elements and the identification of species forming such bonds is remarkably interesting particularly in diatomic anions for which there is a lack of information. Here, it is found that the MX - anions, TcN - , RuC - , RhB - , and PdBe - , present quadruple bonding, as do the corresponding MX neutrals, even though a different type of σ 2 bond is involved in Σ + states of neutral and anions. Specifically, the ground states (X 2 Δ or X 2 Σ + ) of the four anions and their first excited states (A 2 Σ + or A 2 Δ) of TcN - , RuC - , and RhB - present quadruple bonds consisting of two σ and two π bonds: (4d z2 - 2p z ) 2 , 5p z 0  2s 2 , (4d xz - 2p x ) 2 , and (4d yz - 2p y ) 2 . Bond lengths, dissociation energies, spectroscopic data and electron affinities were calculated via high-level multireference and coupled-cluster methodol- ogy using the aug–cc–pV5Z X (-PP) M basis set. Strong bonding results in short bond lengths ranging from 1.602 (TcN - ) to 1.944 (PdBe - ) Å. Adiabatic (diabatic) binding energies reach up to 139 (184) kcal/mol. Electron affinities (EA) were calculated at 1.368 (TcN), 1.242 (RuC), 0.873 (RhB), 0.743 (PdBe) eV. Only for RhB has EA been measured experimentally at 0.961 eV, in good agreement with the value reported here. KEYWORDS coupled cluster calculations, multireference calculations, PdBe - , quadruple bonding, RhB - , RuC - , TcN - 1 | INTRODUCTION Quadruple bonding is very rare for main group elements and recognizing species forming such bonds can advance both the basic interpretation of bonding of diatomic species and the examination of other potential spe- cies forming such bonds. Additionally, the identification of a quadruple bond motif is remarkably interesting, particularly in diatomic anions of transition metals where there is a surprising lack of information in the lit- erature. Furthermore, there is one measured experimental electron affin- ity (EA) for only one of the four calculated species which is in good agreement with the calculated EA; for the remaining species, this study will provide experimentalists with useful data for future studies. Transition metals have very interesting properties which result from their partially occupied d orbitals with loosely bound electrons. These metals are very hard and malleable; they have high melting and boiling points, high electrical and thermal conductivity; they form colored compounds due to d-d electronic transitions. They often exhibit high catalytic activity and tend to form paramagnetic compounds because of the unpaired d electrons [1] . These properties are consequences of the nature of their chemical bonding, hence their study is a very active area of research [2–8] . The analysis of the chemical bonding is a very fundamental aspect of chemistry. The study of quadruple bonding in diatomic species involving main group elements attracts researchers' interest. For instance, the exact multiplicity of the bond of the C 2 molecule was carefully examined and analyzed by many theoretical groups [9] . Fur- thermore, chemical bonding in diatomic anions in particular has not received the attention it deserves. This study is trying to fill this gap by looking at the formation of quadruple bonds in diatomic anions. Revised: 4 March 2021 Accepted: 23 March 2021 DOI: 10.1002/jcc.26527 1126 © 2021 Wiley Periodicals LLC. J Comput Chem. 2021;42:1126–1137. wileyonlinelibrary.com/journal/jcc