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

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 X-3 Delta (TcN) and X-1 Sigma(+) (RuC, RhB, and PdBe) and in the two lowest excited states of TcN (1 Sigma(+), (1)Delta), RuC ((1,3)Delta), and RhB ((1,3)Delta). The quadruple bonds consist of two pi and two sigma bonds: (4d(xz)-2p(x))(2), (44(yz)-2p(y))(2), (4d(z)2-2p(z))(2), and 5s(0) <- 2s(2) (1 Sigma(+)) or 5p(z)(0) <- 2s(2) ((1,3)Delta). 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-pV5Z(X)(-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.