A Quantum Chemical Study of the Quintuple Bond between Two Chromium Centers in [PhCrCrPh]: trans‐Bent versus Linear Geometry

Abstract

The recent report of a stable, crystalline compound with a quintuply bonded chromium-chromium core [1] may reopen the debate on the nature of the multiple bonds between transition metals [2] that was initiated by the discovery of a stable, quadruple bond by Cotton et al. in 1964. [3] Although a recent study of a quintuple bond in the uranium dimer [4] U 2 provided strong support for the existence of such bonds, the isolation of the stable [Ar'CrCrAr'] species (Ar' = 2,6-(2,6-iPr 2 -C 6 H 3 ) 2 -C 6 H 3 ), which has a planar, trans-bent rather than a linear C ipso CrÀCrC ipso core and weak temperature-independent paramagnetism, has raised new questions about multiple bonding. Such a trans-bent structure was recently predicted for the simple [HCrCrH] dimer by Weinhold and Landis on the basis of DFT calculations and a natural-bond analysis. [5] The simplest molecule containing a CrÀCr bond is Cr 2 . Its experimental bond length, obtained from laser-induced fluorescence spectroscopy, is 1.679 , and this extremely short CrÀCr bond has a formal bond order of six, although its significance was questioned [6] because of its low dissociation energy (1.53 AE 0.06 eV). According to different theoretical studies, the description of the CrCr interaction in Cr 2 ranges from a sextuple bond, [7][8][9][10] through a single bond, [11] to a complete absence of bonding between the chromium atoms. [6] It is now well established that a proper description of the bonding in this species requires a multiconfigurational treatment. A description of the bonding in Cr 2 that is in full agreement with the experimental data was obtained in a recent study.[12] Formally, a sextuple bond is formed, but the effective bond order is only about four because of mixing of excited, less-bonding states into the ground state. Similar behavior was also observed for [Re 2 Cl 8 ] 2À , in which a formal quadruple bond was found to have a bond order of only about three for similar reasons. [13,14] The bonding scheme in the [Ar'CrCrAr'] complex resembles that found in the Cr 2 dimer: The interaction of two Cr I centers with d 5 electron configurations leads to five, rather than six, metal-metal bonding molecular orbitals, along with their antibonding counterparts. The questions that arise are: which electronic configurations dominate the ground-state wave function, and how important is the contribution of the bonding (s g ) 2 (p u ) 4 (d g ) 4 configuration? The main difference between the bonding in Cr 2 and [Ar'CrCrAr'] lies in the fact that two additional metal-ligand orbital combinations involving the participation of mainly 4 s orbitals in the metalcarbon(ligand) bond are present in [Ar'CrCrAr']. Despite this apparent similarity, the situation in the real complex is more complicated because of the size of the ligands, and also because of the presence of the "indirect" metal-ligand interactions. Because of the presence of the flanking aryl groups in the vicinity of the chromium centers, a weak, but non-negligible interaction occurs.[1] This interaction, ...

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

“…[15,21] is equal to 3.52 and in this case the closed shell configuration has a weight of only 45 %. In the Mo À Mo and W À W compounds the calculated metal-metal bond order is about one unit larger than in the corresponding Cr À Cr compound.…”

“…Experimental and theoretical works have also shown that [Ar'-CrCr-Ar'] (3 a, Scheme 1) (Ar' = 2,6-(2,6-iPr 2 -C 6 H 3 ) 2 -C 6 H 3 ) features a trans-bent geometry. [15,[19][20][21] Since the Ar ligand successfully stabilizes dimers of main group elements and the Cr dimer, its capabilities in protecting dimers of Fe and Co were also investigated. [22,23] The flanking aryl/metal h 6 interaction makes the FeÀFe and CoÀCo bonds longer than in other compounds.…”

“…We have studied metal-metal multiple bonds in the Cr 2 , Mo 2 , and W 2 dimers by making use of the concept of effective bond order (EBO) [14,15] that arises from a multiconfigurational complete active space-SCF (CASSCF) wavefunction. [16] We have demonstrated that a sextuple bond exists in Mo 2 and W 2 , but hardly in Cr 2 .…”

Progress and Challenges in the Calculation of Electronic Excited States

“…The beautiful work of Gagliardi and others about elucidating multiple bonding in late transition metals (e.g. the quintuple bond between two chromium centers), [134][135][136][137][138] lanthanides [139] and actinides (e.g. the quintuple bond in the uranium molecule) [140,141] is a good example of it.…”

Exploring the Actinide—Actinide Bond: Theoretical Studies of the Chemical Bond in Ac2, Th2, Pa2, and U2

“…19 More strongly quintuply bound diatoms of transition metals may exist. A strong candidate is Nb 2 because Nb has a 4d 4 s 1 ground state in contrast to V and Ta.…”