The nature of metal–metal bond of the dimetallocene complexes [M2(η5-C5R5)2] (M=Zn, Cd, Hg; R=H, Me): An energy decomposition analysis

A lithium-aluminium heterobimetallic dimetallocene

Homobimetallic dimetallocenes exhibiting two identical metal atoms sandwiched between two η5 bonded cyclopentadienyl rings is a narrow class of compounds, with representative examples being dizincocene and diberyllocene. Here we report the synthesis and structural characterization of a heterobimetallic dimetallocene, accessible through heterocoupling of lithium and aluminylene fragments with pentaisopropylcyclopentadienyl ligands. The Al-Li bond features a high ionic character and profits from attractive dispersion interactions between the isopropyl groups of the cyclopentadienyl ligands. A key synthetic step is the isolation of a cyclopentadienylaluminylene monomer, which also enables the structural characterization of this species. In addition to their structural authentication by single-crystal X-ray diffraction analysis, both compounds were characterized by multinuclear NMR spectroscopy in solution and in the solid state. Furthermore, reactivity studies of the lithium-aluminium heterobimetallic dimetallocene with an N-heterocyclic carbene and different heteroallenes were performed and show that the Al-Li bond is easily cleaved.

Quantitative and Chemically Intuitive Evaluation of the Nature of M-L Bonds in Paramagnetic Compounds: Application of EDA-NOCV Theory to Spin Crossover Complexes

To improve understanding of M-L bonds in 3d transition metal complexes, analysis by energy decomposition analysis and natural orbital for chemical valence model (EDA-NOCV) is desirable as it provides a full, quantitative and chemically intuitive ab initio description of the M-L interactions. In this study, a generally applicable fragmentation and computational protocol was established and validated by using octahedral spin crossover (SCO) complexes, as the transition temperature (T1/2 ) is sensitive to subtle changes in M-L bonding. Specifically, EDA-NOCV analysis of Fe-N bonds in five [FeII (Lazine )2 (NCBH3 )2 ], in both low-spin (LS) and paramagnetic high-spin (HS) states led to: 1) development of a general, widely applicable, corrected M+L6 fragmentation, tested against a family of five LS [FeII (Lazine )3 ](BF4 )2 complexes; this confirmed that three Lazine are stronger ligands (ΔEorb,σ+π =-370 kcal mol-1 ) than 2 Lazine +2 NCBH3 (=-335 kcal mol-1 ), as observed. 2) Analysis of Fe-L bonding on LS→HS, reveals more ionic (ΔEelstat ) and less covalent (ΔEorb ) character (ΔEelstat :ΔEorb 55:45 LS→64:36 HS), mostly due to a big drop in σ (ΔEorb,σ ↓50 %; -310→-145 kcal mol-1 ), and a drop in π contributions (ΔEorb,π ↓90 %; -30→-3 kcal mol-1 ). 3) Strong correlation of observed T1/2 and ΔEorb,σ+π , for both LS and HS families (R2 =0.99 LS, R2 =0.95 HS), but no correlation of T1/2 and ΔΔEorb,σ+π (LS-HS) (R2 =0.11). Overall, this study has established and validated an EDA-NOCV protocol for M-L bonding analysis of any diamagnetic or paramagnetic, homoleptic or heteroleptic, octahedral transition metal complex. This new and widely applicable EDA-NOCV protocol holds great promise as a predictive tool.

Mechanistic Insights into the Ni-Catalyzed Reductive Carboxylation of C-O Bonds in Aromatic Esters with CO2 : Understanding Remarkable Ligand and Traceless-Directing-Group Effects

The mechanism of the Ni⁰ -catalyzed reductive carboxylation reaction of C(sp² )-O and C(sp³ )-O bonds in aromatic esters with CO₂ to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C-O bond cleavage, reductive elimination, and/or CO₂ insertion. Of these steps, C-O bond cleavage was found to be rate-determining, and it occurred through either oxidative addition to form a Ni^II intermediate, or a radical pathway that involved a bimetallic species to generate two Ni^I species through homolytic dissociation of the C-O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp² )-O and C(sp³ )-O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni⁰ catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni^I species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C-O bonds were also employed to investigate several experimentally observed phenomena, including ligand-dependent reactivity and site-selectivity.

Electronic structure and bonding of the dinuclear metal M2(CO)10 decacarbonyls: applications of natural orbitals for chemical valence

The nature of the chemical metal-metal bond in M₂(CO)₁₀ (M = Mn, Re, Tc) dinuclear decacarbonyls complexes was investigated for the first time using the natural orbital chemical valence (NOCV) approach combined with the extended transition state (ETS) for energy decomposition analysis (EDA). The optimized geometries carried out at different levels of theory BP86, BLYP, BLYPD and BP86D, showed that the latter method, i.e., BP86D, led to the best agreement with X-ray experimental measurements. The BP86D/TZP results revealed that the computed covalent contribution to the metal-metal bond are 60.5%, 54.1% and 52.0% for Mn-Mn, Re-Re and Tc-Tc, respectively. The computed total interaction energies resulting from attractive terms (ΔE _orb and ΔE _eles), correspond well to experimental predictions, based on bond lengths and energy interaction analysis for the studied complexes.

Zn···Zn interactions at nickel and palladium centers

The analogy between ZnR fragments and the hydrogen radical represents a fruitful concept in organometallic synthesis. The organozinc(ii) and -zinc(i) sources ZnMe₂ (Me = methyl) and [Zn₂Cp*₂] (Cp* = pentamethylcyclopentadienyl) provide one-electron fragments ·ZnR (R = Me, Cp*), which can be trapped by transition metal complexes [L _a M], yielding [L _b (ZnR) _n ]. The addition of the dizinc compound [Zn₂Cp*₂] to coordinatively unsaturated [L _a M] by the homolytic cleavage of the Zn-Zn bond can be compared to the classic oxidative addition reaction of H₂, forming dihydride complexes [L _a M(H)₂]. It has also been widely shown that dihydrogen coordinates under preservation of the H-H bond in the case of certain electronic properties of the transition metal fragment. The σ-aromatic triangular clusters [Zn₃Cp*₃]⁺ and [Zn₂CuCp*₃] may be regarded as the first indication of this so far unknown, side-on coordination mode of [Zn₂Cp*₂]. With this background in mind the question arises if a series of complexes featuring the Zn₂M structural motif can be prepared exhibiting a (more or less) intact Zn-Zn interaction, i.e. di-zinc complexes which are analogous to non-classical dihydrogen complexes of the Kubas type. In order to probe this idea, a series of interrelated organozinc nickel and palladium complexes and clusters were synthesized and characterized as model compounds: [Ni(ZnCp*)(ZnMe)(PMe₃)₃] (1), [Ni(ZnCp*)₂(ZnMe)₂(PMe₃)₂] (2), [{Ni(CN ^t Bu)₂(μ₂-ZnCp*)(μ₂-ZnMe)}₂] (3), [Pd(ZnCp*)₄(CN ^t Bu)₂] (4) and [Pd₃Zn₆(PCy₃)₂(Cp*)₄] (5). The dependence of Zn···Zn interactions as a function of the ligand environments and the metal centers was studied. Experimental X-ray crystallographic structural data and DFT calculations support the analogy between dihydrogen and dizinc transition metal complexes.

When does carbonylation of carbenes yield ketenes? A theoretical study with implications for synthesis

Quantum-chemical calculations using DFT and ab initio methods have been carried out for 32 carbenes RR'C which comprise different classes of compounds and the associated ketenes RR'C═C═O. The calculated singlet-triplet gaps ΔE(S-T) of the carbenes exhibit a very high correlation with the bond dissociation energies (BDEs) of the ketenes. An energy decomposition analysis of the RR'C-CO bond using the triplet states of the carbene and CO as interacting fragments supports the assignment of ΔE(S-T) as the dominant factor for the BDE but also shows that the specific interactions of the carbene may sometimes compensate for the S/T gap. The trend of the interaction energy ΔE(int) values is mainly determined by the Pauli repulsion between the carbene and CO. The stability of amino-substituted ketenes strongly depends on the destabilizing conjugation between the nitrogen lone-pair orbital and the ketene double bonds. There is a ketene structure of the unsaturated N-heterocyclic carbene parent compound NHC1 with CO as a local energy minimum on the potential-energy surface. However, the compound NHC1-CO is thermodynamically unstable toward dissociation. The saturated homologue NHC2-CO has only a very small bond dissociation energy of D(e) = 3.2 kcal/mol. The [3]ferrocenophane-type compound FeNHC-CO has a BDE of D(e) = 16.0 kcal/mol.

Theoretical studies of An(II)(2)(C(8)H(8))(2) (An = Th, Pa, U, and Np) complexes: the search for double-stuffed actinide metallocenes

Complexes of the form An(2)(C(8)H(8))(2) (An = Th, Pa, U, and Np) were investigated using density functional theory with scalar-relativistic effective core potentials. For uranium, a coaxial isomer with D(8h) symmetry is found to be more stable than a C(s) isomer in which the dimetal unit is perpendicular to the C(8) ring axis. Similar coaxial structures are predicted for Pa(2)(C(8)H(8))(2) and Np(2)(C(8)H(8))(2), while in Th(2)(C(8)H(8))(2), the C(8)H(8) rings tilt away from the An-An axis. Going from Th(2)(C(8)H(8))(2) to Np(2)(C(8)H(8))(2), the An-An bond length decreases from 2.81 A to 2.19 A and the An-An stretching frequency increases from 249 to 354 cm(-1). This is a result of electrons populating An-An 5f pi- and delta-type bonding orbitals and varphi nonbonding orbitals, thereby increasing in An-An bond order. U(2)(C(8)H(8))(2) is stable with respect to dissociation into U(C(8)H(8)) monomers. Disproportionation of U(2)(C(8)H(8))(2) into uranocene and the U atom is endothermic but is slightly exothermic for uranocene plus (1)/(2)U(2), suggesting that it might be possible to prepare double stuffed uranocene if suitable conditions can be found to avoid disproportionation.

The tri-mu-hydrido-bis[(eta(5)-C (5)Me (5))aluminum(III)] theoretical study, the assets of sandwiched M(2)H (3) (M of 13th group elements) stability

The stability of the tri-mu-hydrido-bis[(eta(5)-C(5)Me(5))aluminum], Cp*(2)Al(2)H(3), 1 is studied at B3LYP/6-311+G(d,p), CCSD(T)//B3LYP/6-311+G(d,p) and MP4//B3LYP/6-311+G(d,p) levels. The coordination between Al(2)H(3) entity and both C(5)(CH(3))(5) groups is ensured by strong electrostatic and orbital interactions. The orbital analysis of the interacting fragments shows that Al(2)H(3) acceptor, which keeps its tribridged structure, implies the vacant [Formula: see text] and five antibonding ([Formula: see text], e' and e'') molecular orbitals to interact with two orbitals mixtures, b(1) and e" of the donors (C(5)Me(5)). When we take into account the solvent effect, the computation shows that 1 seems to be stable in condensed phase with a tribridged bond between the Al atoms [Cp*Al(micro-H)(3)AlCp*], whereas in the gas phase, the monobridged Cp*AlH(micro-H)AlHCp* 4 is slightly favored (4 kcal mol(-1)). We propose that 1 could be prepared thanks to Cp*Al (2) and Cp*AlH(2) (3) reaction in acidic medium. The experimental treatment of this type of metallocenes would contribute to the development of the organometallic chemistry of 13th group elements.

One-electron metal-metal bond stabilized in dinuclear metallocenes: theoretical prediction of DBe-LiCp (D = C5H5 or C5Me5)

Recently, stimulated by the unexpected synthesis and isolation of a bis-metallic sandwich compound Cp*ZnZnCp* (Cp* = eta(5)-C(5)Me(5)), many studies have focused on various dinuclear metallocenes involving a direct metal-metal (single or multiple) bond. However, we are not aware of any report on the metallocenes incorporating a "one-electron metal-metal bond". Herein, through the good steric and electronic stabilization effect of Cp and Cp*, we for the first time theoretically design a new type of sandwich-like compounds DBe-LiCp (D = Cp or Cp*) associated by an "unaided" one-electron metal-metal bond. Bonding characteristics of CpBe-LiCp were analyzed by natural bond orbital (NBO) theory. To shed more light on the stability of sandwich complexes, the dissociation energies (DBe-LiCp --> DBe + CpLi) and extrusion energies (DBe-LiCp --> DBeCp + Li) were calculated. Through calculation of thermodynamic standard entropies, we predict that these new compounds may be detected in the gaseous phase at appropriate experiment conditions.

Direct bonds between metal atoms: Zn, Cd, and Hg compounds with metal-metal bonds

The synthesis and characterization of [Zn(2)(eta(5)-C(5)Me(5))(2)], a stable molecular compound with a Zn-Zn bond and the first example of a dimetallocene structure, has opened a new chapter in the organometallic chemistry of zinc and in metallocene chemistry. The existence of two directly bonded zinc atoms demonstrates that the [Zn-Zn](2+) unit, the lightest Group 12 homologue of the well-known [Hg-Hg](2+) ion, can be stabilized by appropriate ligands. Activity in this area has increased enormously in the few years since the determination of the structure of this molecule. Numerous theoretical studies have been devoted to the investigation of the electronic, structural, and spectroscopic properties of this and related compounds, and new metal-metal coordination and organometallic compounds of zinc, cadmium, and mercury have been synthesized and structurally characterized. This Minireview gives an overview of activity in this field during the past three to four years.

Synthesis and characterization of the homologous M-M bonded series Ar'MMAr' (M = Zn, Cd, or Hg; Ar' = C6H3-2,6-(C6H3-2,6-Pr(i)2)2) and related arylmetal halides and hydride species

The synthesis and structural characterization of the first homologous, molecular M-M bonded series for the group 12 metals are reported. The compounds Ar'MMAr' (M = Zn, Cd, or Hg; Ar' = C(6)H(3)-2,6-(C(6)H(3)-2,6-Pr(i)(2))(2)) were synthesized by reduction of the corresponding arylmetal halides by alkali metal/graphite (Zn or Hg) or sodium hydride (Cd). These compounds possess almost linear C-M-M-C core structures with two-coordinate metals. The observed M-M bonds distances were 2.3591(9), 2.6257(5), and 2.5738(3) A for the zinc, cadmium, and mercury species, respectively. The shorter Hg-Hg bond in comparison to that of Cd-Cd is consistent with DFT calculations which show that the strength of the Hg-Hg bond is greater. The arylmetal halides precursors (Ar'MI)(1 or 2), and the highly reactive hydrides (Ar'MH)(1 or 2), were also synthesized and fully characterized by X-ray crystallography (Zn and Cd) and multinuclear NMR spectroscopy. The arylzinc and arylcadmium iodides have iodide-bridged dimeric structures, whereas the arylmercury iodide, Ar'HgI, is monomeric. The arylzinc and arylcadmium hydrides have symmetric (Zn) or unsymmetric (Cd) mu-H-bridged structures. The Ar'HgH species was synthesized and characterized by spectroscopy, but a satisfactory refinement of the structure was precluded by the contamination of monomeric Ar'HgH by Ar'H. It was also shown that the decomposition of Ar'Cd(mu-H)(2)CdAr' at room temperature leads to the M-M bonded Ar'CdCdAr', thereby supporting the view that the reduction of the iodide proceeds via the hydride intermediate.