Covalent vs Charge-Shift Nature of the Metal–Metal Bond in Transition Metal Complexes: A Unified Understanding

Abstract

We present here a general conceptualization of the nature of metal–metal (M–M) bonding in transition-metal (TM) complexes across the periods of TM elements, by use of ab initio valence-bond theory. The calculations reveal a dual-trend: For M–M bonds in groups 7 and 9, the 3d-series forms charge-shift bonds (CSB), while upon moving down to the 5d-series, the bonds become gradually covalent. In contrast, M–M bonds of metals having filled d-orbitals (groups 11 and 12) behave oppositely; initially the M-M bond is covalent, but upon moving down the Periodic Table, the CSB character increases. These trends originate in the radial-distribution-functions of the atomic orbitals, which determine the compactness of the valence-orbitals vis-à-vis the filled semicore orbitals. Key factors that gauge this compactness are the presence/absence of a radial-node in the valence-orbital and relativistic contraction/expansion of the valence/semicore orbitals. Whenever these orbital-types are spatially coincident, the covalent bond-pairing is weakened by Pauli-repulsion with the semicore electrons, and CSB takes over. Thus, for groups 3–10, which possess (n – 1)­s2(n – 1)­p6 semicores, this spatial-coincidence is maximal at the 3d-transition-metals which consequently form charge-shift M–M bonds. However, in groups 11 and 12, the relativistic effects maximize spatial-coincidence in the third series, wherein the 5d10 core approaches the valence 6s orbital, and the respective Pauli repulsion generates M–M bonds with CSB character. These considerations create a generalized paradigm for M–M bonding in the transition-elements periods, and Pauli repulsion emerges as the factor that unifies CSB over the periods of main-group and transition elements.

A Crystalline Tri-thorium Cluster: When Metal σ-Aromaticity Just Isn’t Enough

“…However, an additional in-depth investigation reveals that the thorium atoms put up a united front with the surrounding chlorine atoms to form an unprecedented multicenter charge-shift bonding pattern, which is demonstrated to be the primary factor underlying the high symmetry and remarkable stability of the tri-thorium cluster 3". The charge-shift bonds have recently been shown to be particularly strong when the 5d orbitals of the transition-metals from group 11 and 12 are involved, 15 but this study extends the range of the charge-shift bonding beyond transition metals to a high of principal quantum number six and to the seventh row of the periodic table.…”

Valence Bond Theory—Its Birth, Struggles with Molecular Orbital Theory, Its Present State and Future Prospects

“…Subsequently, Wu et al extended the spin-free approach, and produced a general-purpose VB program initially called the XIAMEN-99 package, and more recently named XMVB [113,114]. XMVB is becoming faster and more efficient every year, and its VBSCF routine can include up to 26 orbitals/26 electrons in the VB space [97,115], and can also handle molecules with two transition metals and as many as 10 ligands like CO [116]. The new XMVB versions also have DFVB methods, which combine DFT and VBT [117].…”

“…The generation of so many different VB methods has advantages as well as some less productive byproducts. Thus, the advent of a number of good VB programs has caused a surge of applications of VB theory, to problems ranging from bonding in main group elements to transition metals [116], conjugated systems, aromatic and antiaromatic species, all the way to excited states and full pathways of chemical reactions, with moderate to very good accuracies. For example, a recent calculation of the barrier for the identity hydrogen exchange reaction, H + H-H' → H-H + H', by Song et al [132] shows that it is possible to calculate the reaction barrier accurately with just eight classical VB structures.…”

Self-healing Materials: A Review of Recent Developments

“…When a coordination bond is formed, it is no different from a common covalent bond, except that the two electrons shared between the bonding atoms do not come from one atom, but from the other, and the metal coordination bond is the ligand and metal atoms or ions formed by the coordination bond. [67] Metal Coordination Bond also has the characteristics of covalent bond: directivity and saturation, [68] at the same time, metal coordination bond has certain conductivity [69] to make it more widely used. In recent years, it has been widely used in the field of self-healing to improve the performance of selfhealing materials, so that the materials have better self-healing performance, [70][71][72] preeminent electrical conductivity.…”

“…Ionic bonds [96,97] are formed after two or more atoms or chemical groups lose or gain electrons and become ions, scilicet, positive and negative ions between the electrostatic gravity formed by the chemical bond. The ionic bond not only has strong force, but also has the characteristics of nonsaturation and non-direction, in contrast, ionic bonds are nearly as strong as covalent bonds, but stronger than hydrogen bonds.…”