Assessing Metal–Metal Multiple Bonds in CrCr, MoMo, and WW Compounds and a Hypothetical UU Compound: A Quantum Chemical Study Comparing DFT and Multireference Methods

Abstract

To gain insights into the trends in metal-metal multiple bonding among the Group 6 elements, density functional theory has been employed in combination with multiconfigurational methods (CASSCF and CASPT2) to investigate a selection of bimetallic, multiply bonded compounds. For the compound [Ar-MM-Ar] (Ar=2,6-(C(6)H(5))(2)-C(6)H(3), M=Cr, Mo, W) the effect of the Ar ligand on the M(2) core has been compared with the analogous [Ph-MM-Ph] (Ph=phenyl, M=Cr, Mo, W) compounds. A set of [M(2)(dpa)(4)] (dpa=2,2'-dipyridylamide, M=Cr, Mo, W, U) compounds has also been investigated. All of the compounds studied here show important multiconfigurational behavior. For the Mo(2) and W(2) compounds, the σ(2)π(4)δ(2) configuration dominates the ground-state wavefunction, contributing at least 75%. The Cr(2) compounds show a more nuanced electronic structure, with many configurations contributing to the ground state. For the Cr, Mo, and W compounds the electronic absorption spectra have been studied, combining density functional theory and multireference methods to make absorption feature assignments. In all cases, the main features observed in the visible spectra may be assigned as charge-transfer bands. For all compounds investigated the Mayer bond order (MBO) and the effective bond order (EBO) were calculated by density functional theory and CASSCF methods, respectively. The MBO and EBO values share a similar trend toward higher values at shorter normalized metal-metal bond lengths.

Metal–Metal Single Bonds with the Magnetic Anisotropy of Quadruple Bonds: A Systematic Series of Heterobimetallic Bismuth(II)–Rhodium(II) Formamidinate Complexes

“…If we only consider SOC, we would expect the trend in |Δ χ | to be Cr 2 <Mo 2 <W 2 , which does explain why the W 2 molecule has the largest | Δ χ| . However, for the Cr 2 molecule, Δ E becomes important, because it is known that Cr 2 quadruple bonds are weak and have several low‐lying excited states . This Δ E analysis likely contributes to the large Δ χ values of all first‐row metal dimers.…”

Electronic Structure of Ru2(II,II) Oxypyridinates: Synthetic, Structural, and Theoretical Insights into Axial Ligand Binding

“…98 The FSR of the various adducts increases as CO ≪ N 2 < S-DMSO ≈ PPh 3 < MeCN < py < NH 3 < O−DMSO < THF ≈ Ru 2 (chp) 4 (with or without ZnCl 2 ) < H 2 O < CH 2 Cl 2 . This list is nearly the reverse of the sequence of bond orders, which in most cases reflects the expected relative π-withdrawing/donating character of the axial ligand.…”

Is it True That the Normal Valence‐Length Correlation Is Irrelevant for Metal–Metal Bonds?

“…for the quadruple Re−Re bond in K 2 Re 2 Cl 8 ⋅H 2 O . Numerous theoretical studies have focused mainly on the ability of different quantum chemistry approaches (e.g., Mulliken approximation, Natural Bond Order analysis, domain averaged Fermi holes analysis) to describe EBOs that provide useful chemical insight . Structural reasons for the reduction in bond order were in many cases not addressed, although some notable exceptions exist .…”