Molecular Hume–Rothery Compounds [M(ZnR)n] and [M(ZnR)a(GaR)b] (a + 2b = n ≥ 8): Relations of Coordination Polyhedra and Electronic Structure

Homoleptic hexa- and penta-coordinated gallium(I) amide complexes of ruthenium and molybdenum

Reaction of neutral olefin complexes of ruthenium and molybdenum with GaTMP (TMP = 2,2,6,6-tetramethylpiperidinyl) by substitution leads to the formation of respective five- and six-coordinated homoleptic products. [Ru(GaTMP)5] (1) and [Mo(GaTMP)6] (2) were isolated and characterized. Core structure geometries were analyzed using continuous shape measure, and the complexes were subjected to DFT calculations unveiling competing π-interactions between the transition metal center and the amido substituent with the unoccupied pπ orbitals of the gallium.

Formation of a Propeller-Shaped Ni4Ga3 Cluster Supported by Transmetalation of Cp* from Ga to Ni

The reactivity of GaCp* toward different Ni⁰ olefin complexes is investigated. The reaction of GaCp* with [Ni(cdt)] (cdt = all-trans-1,5,9-cyclododecatriene) leads to simple adduct formation and the 18 valence electron (ve) compound [Ni(GaCp*)(cdt)] (1). In contrast, [Ni₂(dvds)₃] (dvds = 1,1,3,3-tetramethyl-1,3-divinyldisiloxane) is converted to the undercoordinated and highly reactive 16 ve complex [Ni(GaCp*)(dvds)] (2), which represents an intermediate in the formation of the propeller-shaped M₇ cluster [Ni₄Ga₃](Cp*)₃(dvds)₂ (3). Extensive characterization of the latter compound by experimental and computational means reveals the Cp* transfer from Ga to Ni. Therefore, the title compound can be best expressed by the structural formula [(μ₂-GaCp*)(Ni₂)(μ₂-GaNiCp*)₂(dvds)₂]. The flexible dvds ligands stabilize this arrangement via alkene-Ni and O-Ga interactions. Furthermore, compound 2 exhibits a fast GaCp* ligand exchange with external GaCp*, which is rather unexpected for the [TM(ECp*)_a] compounds; they usually do not undergo substitution reactions with two electron donor ligands like CO, phosphines, or GaCp*.

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.

Hume-Rothery phase-inspired metal-rich molecules: cluster expansion of [Ni(ZnMe)₆(ZnCp*)₂] by face capping with Ni⁰(η⁶-toluene) and Ni(I)(η⁵-Cp*)

The novel all-hydrocarbon ligand-stabilized binuclear clusters of metal-core composition Ni2Zn7E, [(η(5)-Cp*)Ni2(ZnMe)6(ZnCp*)(ECp*)] (1-Zn, E = Zn; 1-Ga, E = Ga) and [(η(6)-toluene)Ni2(ZnCp*)2(ZnMe)6] (2; Cp* = pentamethylcyclopentadienyl), were obtained via Ga/Zn and Al/Zn exchange reactions using the starting compounds [Ni2(ECp*)3(η(2)-C2H4)2] (E = Al/Ga) and an excess of ZnMe2 (Me = CH3). Compounds 1-Zn and 1-Ga are very closely related and differ only by one Zn or Ga atom in the group 12/13 metal shell (Zn/Ga) around the two Ni centers. Accordingly, 1-Zn is EPR-active and 1-Ga is EPR-silent. The compounds were derived as a crystalline product mixture. All new compounds were characterized by (1)H and (13)C NMR and electron paramagnetic resonance (EPR) spectroscopy, mass spectrometric analysis using liquid-injection field desorption ionization, and elemental analysis, and their molecular structures were determined by single-crystal X-ray diffraction studies. In addition, the electronic structure has been investigated by DFT and QTAIM calculations, which suggest that there is a Ni1-Ni2 binding interaction. Similar to Zn-rich intermetallic phases of the Hume-Rothery type, the transition metals (here Ni) are distributed in a matrix of Zn atoms to yield highly Zn-coordinated environments. The organic residues, ancillary ligands (Me, Cp*, and toluene), can be viewed as the "protecting" shell of the 10-metal-atom core structures. The soft and flexible binding properties of Cp* and transferability of Me substituents between groups 12 and 13 are essential for the success of this precedence-less type of cluster formation reaction.

Oligonuclear molecular models of intermetallic phases: a case study on [Pd2Zn6Ga2(Cp*)5(CH3)3]

The synthesis, characterization, and theoretical investigation by means of quantum-chemical calculations of an oligonuclear metal-rich compound are presented. The reaction of homoleptic dinuclear palladium compound [Pd(2)(μ-GaCp*)(3)(GaCp*)(2)] with ZnMe(2) resulted in the formation of unprecedented ternary Pd/Ga/Zn compound [Pd(2)Zn(6)Ga(2)(Cp*)(5)(CH(3))(3)] (1), which was analyzed by (1)H and (13)C NMR spectroscopy, MS, elemental analysis, and single-crystal X-ray diffraction. Compound 1 consisted of two C(s)-symmetric molecular isomers, as revealed by NMR spectroscopy, at which distinct site-preferences related to the Ga and Zn positions were observed by quantum-chemical calculations. Structural characterization of compound 1 showed significantly different coordination environments for both palladium centers. Whilst one Pd atom sat in the central of a bi-capped trigonal prism, thereby resulting in a formal 18-valence electron fragment, {Pd(ZnMe)(2)(ZnCp*)(4)(GaMe)}, the other Pd atom occupied one capping unit, thereby resulting in a highly unsaturated 12-valence electron fragment, {Pd(GaCp*)}. The bonding situation, as determined by atoms-in-molecules analysis (AIM), NBO partial charges, and molecular orbital (MO) analysis, pointed out that significant Pd-Pd interactions had a large stake in the stabilization of this unusual molecule. The characterization and quantum-chemical calculations of compound 1 revealed distinct similarities to related M/Zn/Ga Hume-Rothery intermetallic solid-state compounds, such as Ga/Zn-exchange reactions, the site-preferences of the Zn/Ga positions, and direct M-M bonding, which contributes to the overall stability of the metal-rich compound.