Quadruple Bonding in the Ground and Low-Lying Excited States of the Diatomic Molecules TcN, RuC, RhB, and PdBe

Abstract

Multiple bonds between atoms are one of the most fundamental aspects of chemistry. Double and triple bonds are quite common, while quadruple bonds are a true oddity and very rare for the main group elements. Identifying molecules containing quadruple bonds is very important and, even more so, determining the necessary requirements for the existence of such bonds. Here we present high-level theoretical calculations on the isoelectronic MX molecules, i.e., TcN, RuC, RhB, and PdBe, showing that such a quadruple bond with main group elements is not that uncommon. We found that quadruple bonds are formed in their ground states X3Δ (TcN) and Χ1Σ+ (RuC, RhB, and PdBe) and in the two lowest excited states of TcN (1Σ+, 1Δ), RuC (1,3Δ), and RhB (1,3Δ). The quadruple bonds consist of two π and two σ bonds: (4d xz –2p x )2, (4d yz –2p y )2, (4d z 2 –2p z )2, and 5s0 ← 2s2 (1Σ+) or 5p z 0←2s2 (1,3Δ). Bond lengths, dissociation energies, dipole moments, spectroscopic parameters, and relative energy ordering of the states were calculated via multireference and coupled cluster methodology using the aug-cc-pV5ZX(-PP)M basis sets. We study how the atomic states involved and how the gradual transition from covalent to dative bond, from TcN to PdBe, influence all of the calculated data, such as bond dissociation energies, bond lengths, and relative energy ordering of the states. Finally, we report the requirements for the occurrence of such bonds in molecular systems. All Be, B, C, and N atoms combining with the appropriate second-row transition metal can form quadruple bonds, while they cannot form such bonds with the first-row transition metals.

Quadruple chemical bonding in the diatomic anions TcN−, RuC−, RhB−, and PdBe−

“…The small elongation of bond distance is attributed to the attachment of an electron to a non-bonding orbital. [11], while for the anion is 152.4 (109.9) [109.9] kcal/mol A 2 Σ + (TcN − ) at the MRCISD+Q level of theory; i.e., the electron attachment further destabilizes the Σ + state with respect to adiabatic products, but it stabilizes it by 10 kcal/mol with respect to the ground state products.…”

“…The Χ 2 Σ + and A 2 Δ states correlate to Pd − ( 2 S; 4d 10 5s 1 ) and Pd − ( 2 D; 4d 9 5s 2 ) + Be( 1 S; 2s 2 ), respectively, see Figure 5, and they retain their correlated character in their whole PEC. (52.8 [11] ) and 21.1 (24.2, 29.1) [11] kcal/mol, respectively, see The calculated RCCSD(T) EA of Pd and PdBe are 0.479 and 0.743 eV, respectively. There is a good agreement with the available experimental EA value of Pd, which was measured at 0.557 [27] and 0.56214 eV [32] .…”

“…In 2020, two studies were published on the quadruple bonding of 2nd row transition metals with main group elements [10,11] and one study on the nature of the bonding of 3rd row transition metals [12] . The quadruple bonding in the ground state of RhB was first reported by Cheung et al [10] , while our group [11] found that four molecules, i.e., TcN, RuC, RhB and PdBe (MX), present ground states with a quadruple bond and in the case of TcN, RuC, and RhB, their two lowest excited states also have quadruple bonds. The necessary requirements for the occurrence of quadruple bonds were determined, i.e., (i) the existence of low-lying atomic states that have low lying unoccupied orbitals that can receive electrons via dative bonds and (ii) atoms with doubly occupied orbitals that can form dative bonds.…”

“…It should be noted that while there are experimental and theoretical data on the neutral TcN [11,13,14], RuC [11,13,[15][16][17][18], , RhB [10,11,13,19] and PdBe [11] molecules, the literature on the corresponding anions is very limited, see below. A synopsis of all previous studies on the four neutral MX species is reported in ref.…”

“…A synopsis of all previous studies on the four neutral MX species is reported in ref. [11]. Regarding the MX − anions, there are only three studies on two anions, i.e., on the 2 Σ + state of RuC − [13,15] and RhB − [10,13] .…”