Chemistry of 2-(arylazo)phenolate complexes of ruthenium. Synthesis,structure and reactivities

A group of six ruthenium(III) complexes of type [Ru(acac)(L)₂]where acac=acetylacetonate anion and L=2-(arylazo)-4-methylphenolate anion or 1-(phenylazo)-2-naphtholate anion have been synthesized and characterized Structural characterization of a representative complex where L=1-(phenylazo)-2-naphtholate anionshows that the azophenolate ligands are coordinated as NO-donor ligands forming six-membered chelate rings The complexes are paramagnetic (low-spin d⁵S=1/2) and show rhombic ESR spectra in 1:1 dichloromethane–toluene solution at 77 K In carbon tetrachloride solution these complexes show intense LMCT transitions in the visible region together with weak ligand-field transitions in the near-IR region All the complexes display two cyclic voltammetric responses a ruthenium(III)–ruthenium(IV) oxidation in the range of 083 to 103 V vs SCE and a ruthenium(III)–ruthenium(II) reduction in the range of −024 to −052 V vs SCE Formal potentials of both the couples correlate linearly with the Hammett constant of the para substituent in the arylazo fragment of the 2-(arylazo)-4-methylphenolate ligand The ruthenimn(IV) and ruthenium(II) congeners of the [Ru^III(acac)(L)₂] complexes have been generated by chemical or electrochemical methods and they have been characterized by electronic spectroscopy and cyclic voltammetry.     

A 13C NMR study of some metal acetylacetone complexes

¹³C NMR spectra of nine metal acetylacetonates in (H₃C)₂SO-d₆ and ten metal acetylacetonates in CDCl₃ are reported. The complexes are of the type M(acac)_n where n is 1,2,3 or 4. The downfield shift of the methyl, methine and carbonyl carbon resonances from tetramethylsilane (TMS) are interpreted in terms of the complexed metal and a solvent effect. ¹³C¹⁹⁵Pt spin coupling is also observed for the Pt(acac)₂ complex.     

Electronic Structure and Magnetic Properties of a High-Spin MnIII Complex: [Mn(mesacac)3] (mesacac=1,3-Bis(2,4,6-trimethylphenyl)-propane-1,3-dionato)

Metal acetylacetonates of the general formula [M(acac)₃] (M^III=Cr, Mn, Fe, Co) are among the best investigated coordination compounds. Many of these first-row transition metal complexes are known to have unique electronic properties. Independently, photophysical research with different β-diketonate ligands pointed towards the possibility of a special effect of the 2,4,6-trimethylphenyl substituted acetylacetonate (mesacac) on the electron distribution between ligand and metal (MLCT). We therefore synthesized and fully characterized the previously unknown octahedral title complex. Its solid-state structure shows a Jahn-Teller elongation with two Mn−O bonds of 2.12/2.15 Å and four Mn−O bonds of 1.93 Å. Thermogravimetric data show a thermal stability up to 270 °C. High-resolution mass spectroscopy helped to identify the decomposition pathways. The electronic state and spin configuration of manganese were characterized with a focus on its magnetic properties by measurement of the magnetic susceptibility and triple-zeta density functional theory (DFT) calculations. The high-spin state of manganese was confirmed by the determination of an effective magnetic moment of 4.85 μ_B for the manganese center.     

Molecular complexes of iodine with metal acetylacetonates

The ability of metal acetylacetonates to act as electron donors and form molecular complexes with I₂ was studied by examining the electronic, vibrational, and NMR spectra of the complexes. The specific compounds used in the study were Al(acac)₃ Sc(acac)₃ Zr(acac)₄, and Th(acac)₄. The electronic spectra of mixtures of the metal acetylacetonates with I₂ in CHCl₃ had, in addition to the absorption peaks characteristic of the free components, two peaks that were due to the charge transfer complexes. For each complex, the highest wavelength peak (near 360 nm) was assigned to the blue shifted I₂ band, while the lower peak (between 270 nm and 305 nm) was attributed to the intermolecular charge transfer. In the i.r. spectra of each complex, the major effect of complexation was to cause the I₂ stretching frequency to appear between 145 cm⁻¹ and 160 cm⁻¹. The positions of the absorption peaks in both the electronic and vibrational spectra led to the conclusion that in these complexes, I₂ had received a large amount of charge from the donors. Complex formation had little effect on the NMR spectra of the donors. Association constants of 1:1 complexes were determined from the concentration dependence of the absorbance of the blue shifted I₂ bands. Values of ΔHdg and ΔS°₂₉₈ for the complex formation were obtained from the temperature variation of the association constants. The data indicate that the complexes are extremely stable species. Both the stability of the complexes and the high degree of charge transfer were rationalized by considering a model for the intermolecular interactions that involved two M(acac) rings simultaneously transferring charge from one donor to an I₂ molecule.     

Metallionenaustausch im System Acetylacetonat/Halogenid/Tetrahydrofuran

Exchange of Metal Ions in the System Acetylacetonate/Halide/Tetrahydrofuran . As bifunctional Lewis bases metal bis(acetyl-acetonates) react with zinc(II) chloride under formation of binuclear complexes (THF)₂M(acac)₂ZnCl₂ (M = Ni, Co, Mg). The octahedral and the tetrahedral centre of these compounds are connected by tridentate oxygen atoms of the two acetylacetonato ligands which are simultaneously part of a four-membered ring MOZnO. The addition is combined with a deformation of the octahedral centre, as a prerequisite of a closest package of the atoms within the MOZnO ring. With mercury(II) chloride the metal bis(acetylacetonates) react as tetrafunctional Lewis bases. In the trinuclear complexes (THF)₂M(acac)₂(HgCl₂)₂ (M = Co, Ni, Mg) the interaction between the three coordination centres is weak. No structural change of the octahedral centre (THF)₂M(acac)₂ is found, but the HgCl₂ group diverge slightly from linearity. A ligand exchange was observed in the following cases:

• (a) Reaction of vanadium(III) chloride with Co(acac)₂ under formation of [(THF)₂V(acac)₂][(THF)CoCl₃] (transition of the acetylacetonato ligands to the higher valent central atom).
• (b) Reaction of magnesium halides with M(acac)₂ and formation of (THF)₃Mg(acac)₂MX₂
• (c) Formation von (THF)₃Co(acac)(μ—C1)ZnC1₂ (V) by the reaction of Zn(acac)₂ with cobalt(l1) chloride

In the complex V the octahedral ccntrc of cobalt(II) is connected with the tetrahedral centre of zinc(I1) by tridentate oxygen atom of the acetylacetonato ligand and a chloro bridge (formation of the four-membered CoClZnO cycle with a closest package of the atoms). A driving force for reaction c is the formation of the stable tetrahedral OZnCI₃ group. Important for the li- gand exchange according to c is the stability of the octa- hcdral MgO₆ moiety and the easy formation of the tetra- hedral MO₂X₂ group with a transition metal ion. (THF)₃Co(acac)(μ-CI)(HgCl₂) (VI) is isoslruclural with V. But there is a marked diffcrence between the bond angles of the tetrahedral central atom including the termi- nal chloro ligands (120.7° for V; 143.7° for VI). The crystal structure of (THF)₃Co(acac)(μ-CI)ZnCl₂ (V) was determined by X-ray diffraction: monoclinic; space group P2,/n; Z = 4; a = 1 177.4(5); b = 1628.9(4); c = 1284.2(6) pm; β = 99.54(4)°; R = 6.71 % for 2160 observed reflections.     

Film-Forming Capacity of Sn(II), Zr(IV), and Hf(IV) Acetylacetonates

Zr(acac)₄, Hf(acac)₄, and SnHacacCl₂ · 2H₂O were prepared in the solid state and in enthanol solutions. The film-forming capacity and thermal stability of these compounds were studied. Films of ZrO₂, HfO₂, and SnO₂ were prepared from film-forming solutions of the corresponding acetylacetonates.     

Vanadium complexes with thiosemicarbazones: Synthesis,characterization, crystal structures and anti-Mycobacterium tuberculosis activity

The development of more efficient anti-tuberculosis drugs is of interest. Three oxovanadium(IV) and three cis-dioxovanadium(V) complexes with thiosemicarbazone derivatives bearing moieties with different lipophilicity have been prepared and had their inhibitory activity against Mycobacteriumtuberculosis H₃₇Rv ATCC 27294 evaluated. The analytical methods used by the complexes’ characterization included IR, EPR, ¹H, ¹³C and ⁵¹V NMR spectroscopies, elemental analysis, cyclic voltammetry, magnetic susceptibility measurement and single crystal X-ray diffractometry. [VO(acac)(aptsc)], [VO(acac)(apmtsc)] and [VO(acac)(apptsc)] (acac = acetylacetonate; Haptsc = 2-acetylpyridinethiosemicarbazone; Hapmtsc = 2-acetylpyridine-N(4)-methyl-thiosemicarbazone and Happtsc = 2-acetylpyridine-N(4)-phenyl-thiosemicarbazone) are paramagnetic and their EPR spectra are consistent with the monoanionic N,N,S-tridentate coordination of the thiosemicarbazone ligands, resulting in octahedral structures of rhombic symmetry and with the oxidation state +IV for the vanadium atom. As result of oxidation of the vanadium(IV) complexes above, the diamagnetic cis-dioxovanadium(V) complexes [VO₂(aptsc)], [VO₂(apmtsc)] and [VO₂(apptsc)] are formed. Their ¹H, ¹³C and ⁵¹V NMR spectra were acquired and support a distorted square pyramidal geometry for them, in accord with the solid state X-ray structures determined for [VO₂(aptsc)] and [VO₂(apmtsc)]. In general, the vanadium compounds show comparable or larger anti-M. tuberculosis activities than the free thiosemicarbazone ligands, with MIC values within 62.5–1.56 (μg/mL).     

Good linear relationship between logarithms of Eigen’s water exchange constants for several divalent metal ions and activation energies of corresponding metal-catalyzed alkoxysilane hydrolysis in ethylene-propylene copolymer system

Catalytic efficiencies of seven divalent metal acetylacetonate complexes [M(acac)₂; M = Cd(II), Co(II), Cu(II), Fe(II), Ni(II), Pb(II), and Zn(II)] with respect to the water-crosslinking kinetics of vinyltrimethoxysilane-grafted ethylene-propylene copolymer (EPR-g-VTMS) were investigated to examine the effects of progressive changes in metal ion using ATR-FTIR spectroscopy. The hydrolysis activation energies of EPR-g-VTMS follows the order: No catalyst ≈ Ni(acac)₂ > Co(acac)₂ > Fe(acac)₂ ≈ Zn(acac)₂ > Cd(acac)₂ ≈ Cu(acac)₂ > Pb(acac)₂. Interestingly, the kinetics results revealed that the plots of hydrolysis activation energies of EPR-g-VTMS containing M(acac)₂ complexes and Eigen’s water exchange constants for corresponding metal ions showed a excellent linear relationship, suggesting that the reaction pathway for the silane water-crosslinking with hydrous M(acac)₂ complex in EPR-g-VTMS system may be similar to that for water exchange of the metal ion in an aqueous system. Based on the knowledge of traditional kinetics studies by Eigen and Wilkins and hybrid sol-gel chemistry, the plausible catalytic mechanism for M(acac)₂ complexes in EPR-g-VTMS system was proposed.     

Quinone transfer radical polymerization (QTRP) of styrene: Catalysis by different metal complexes

Styrene has been polymerized by a Quinone Transfer Radical Polymerization (QTRP) based on the redox reaction of an ortho‐quinone and a metal catalyst. Several metal acetylacetonates have been tested in this work. The radical polymerization of styrene is largely controlled when phenanthrenequinone (PhQ) is used with catalytic amounts of Co(acac)₂, Ni(acac)₂, Mn(acac)_{2 or 3}, and Al(acac)₃. As a rule, in the presence of all these metallic complexes, the polystyrene molar mass increases with the monomer conversion, and polydispersity (M_w/M_n) is in the 1.3–1.6 range (at least until 40% monomer conversion). Styrene polymerization has also been resumed by polystyrene chains prepared by QTRP. In the specific case of manganese acetylacetonates, an amine or phosphine ligand has to be added for the control to be effective. Finally, two mechanistic hypotheses have been proposed, depending on whether the oxidation state of the metal can be easily changed or not. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2723‐2733, 2005     

Syntheses,Crystal Structures and Ligand Field Properties of Iron(II) Complexes with PNP Ligands: Origin of Large Ligand Field by a Phosphorous Donor Atom

New iron(II) complexes were synthesized with two tridentate hybrid ligands having phosphorous and nitrogen donor sites, in order to quantitatively estimate the difference of the ligand-field strengths of phosphorous and nitrogen donor sites in cationic metal complexes. Iron(II) complexes with bis(dimethylphosphinoethyl)amine (PNP) and 2,6-bis(diphenylphosphinomethyl)pyridine (PpyP) ligands crystallized as un-symmetric facial-[Fe(PNP)₂](PF₆)₂·CH₃NO₂ and mer-[Fe(PpyP)₂](CF₃SO₃)₂, respectively, as expected from the steric congestion and from the tendency to avoid the mutual trans influence between two phosphorous donor sites. Both complexes are in the low-spin electronic state up to 400 K. The pseudo-D _4h coordination geometry of the PpyP complex made it possible to separate axial (2 × N) and equatorial (4 × P) contributions to the overall ligand-field by means of a spectrometric method: the difference in the ligand-field strengths by the equatorial Ph₂P-donor sites and by the axial 2,6-disubstituted pyridine donor sites is ca. 13,200 cm^?1. A significantly reduced inter-electronic repulsion parameter (425 cm^?1 for both PNP and PpyP complexes) from the value of the free ion (1,060 cm^?1) indicates covalent interaction between the Fe(II) and P atoms even in these cationic metal complexes. It is shown that the degree of covalency as well as the coordination bond strengths between various metal ions and phosphorous/nitrogen donor atoms is successfully explained by the relative energy levels of interacting atomic orbitals calculated on the basis of the Thomas–Fermi–Dirac potential.     

Beiträge zur Chemie der Alkylverbindungen von Übergangsmetallen. 58. Zur Kenntnis von 2-(Dimethylaminomethyl)ferrocenyl-Verbindungen des Vanadiums,Molybdäns,Wolframs, Thoriums und Urans

Contributions to the Chemistry of Transition Metal Alkyl Compounds. 58 On 2-(Dimethylaminomethyl)ferrocenyl Compounds of Vanadium, Molybdenum, Tungsten, Thorium, and Uranium Earlier results according to which dimethylaminomethylferrocenyl groups (FcN) are able to form stable organometallic chelate compounds were confirmed by synthesis of the heterobimetallic chelate compounds (FcN)₂VO · Li(acac) II , (FcN)MoO₂(acac) III , (FcN)WOCl₃ IV , (FcN)Th(acac)₃ V , and (FcN)UO₂(acac) VI from the corresponding metal acetylacetonates or oxidchlorides and (FcN)Li I . The new compounds were characterized by elemental analysis, the i.r., ¹H-n.m.r., and electron spectra and by their effective magnetic moments.     

The infrared spectra (3500-140 cm−1) of the 2,2'-bipyridine, 2-aminomethylpyridine and ethylenediamine adducts and the sodium tris-compounds of cobalt(II), nickel(II) and zinc(II) acetylacetonates

Summary The M(acac)₂B and NaM(acac)₃ complexes (M = Coll, Ni^ll orZn^ll; acac=acetylacetonateanion; B=2,2=bipyridine, 2-aminomethylpyridine or ethylenediamine) have been prepared and their i.r. spectra determined over the 3500-140 cm^–1 range. Complete band assignments have been made on a comparative basis, with the effects of ligand substitution and metal ion substitution being considered. Bands in the i.r. spectra of the complexes which are associated with 2,2-bipyridine, 2-aminomethylpyridine and ethylenediamine arc in excellent agreement with values previously established for these ligand and metal-ligand vibrations, in related complexes. Band assignments for acetylacetonate anion and metal-acetylacetonate vibrations substantiate those made in certain of the previously published papers. The method of assignment adopted has allowed the complete assignment of bands in the i.r. spectra of the metal complexes.     

Die binuklearen Produkte der Umsetzung von Grignardreagenzien mit Acetylacetonaten der späten 3d-Elemente und ihre katalytische Wirkung bei der Kreuzkopplung

Binuclear Reaction Products of Grignard Compounds and Transition Metal Acetylacetonates - Catalysts of Cross Coupling In THF Grignard compounds react with transition metal acetylacetonates forming the binuclear complexes (THF)₂Mg(acac)₂M^IIX₂ (M^II ? Fe, Co, Ni; X ? Cl, Br). A characteristic component of these compounds is the octahedral moiety (THF)₂Mg(acac)₂. The coordination of two cis-positioned oxygen donors of this moiety to an M^IIX₂-group builds up a second (tetrahedral) coordination centre. The structure of the binuclear complexes and their vis-spectra are discussed in detail. The new compounds are cross coupling catalysts of acylhalides and Grignard reagents. Probably in the course of the catalytic process the acylhalides are activated at the unsaturated transition metal centre (increase of the coordination number), but the Grignard reagent at the magnesium centre (substitution of one of the weakly bound THF-ligands). This model also explains the different catalytic activity of (THF)₂Mg(acac)₂FeCl₂ and (THF)₂Mg(acac)₂NiBr₂.     

Electron spin resonance studies on some copper(II) complexes with a few nitrogen donors derived from pyridine

Electron spin resonance spectral studies have been made on copper(II) chloride, bromide, thiocyanate and sulphate complexes with some pyridine derivatives, viz. nicotinic acid (NA), nicotinamide (NICA) and isonicotinamide (INA) in solid and DMF-solution states to see the effect of different anions on the Spin-Hamiltonian parameters at the paramagnetic site for a particular ligand. The spectra of the complexes for a particular anion are almost comparable suggesting the same local symmetry for them. Analysis of the ESR data reveals axial symmetry for all the complexes, except Cu(NA)₂SO₄ for which a rhombic symmetry is suggested. The study shows the interaction of solvent (DMF) molecules with copper(II) ion in the axial plane as evident from the differences in 295 and 77 K g_| values. Moreover, the spectra are consistent with the complete absence or negligbly small copper(II)—copper(II) interactions (in few cases) in these complexes. The various Spin-Hamiltonian parameters calculated from ESR data indicate the presence of an unpaired electron in the d_x²−y² orbital of the copper(II) ion with the additional possibility of a d_xy ground state for Cu(NA)₂SO₄.     

Toward a general strategy for the synthesis of heterobimetallic coordination complexes for use as precursors to metal oxide materials: synthesis, characterization, and thermal decomposition of Bi(2)(Hsal)(6).M(acac)(3) (M = Al, Co, V, Fe, Cr)

Bismuth(III) salicylate, [Bi(Hsal)(3)](n), reacts readily with the trivalent metal beta-diketonate compounds M(acac)(3) (acac = acetylacetonate; M = Al, V, Cr, Fe, Co) to produce trinuclear coordination complexes of the general formula Bi(2)(Hsal)(6).M(acac)(3) (M = Al, V, Cr, Fe, Co) in 60-90% yields. Spectroscopic and single crystal X-ray diffraction experiments indicate that these complexes possess an unusual asymmetric nested structure in both solution and solid state. Upon standing in dichloromethane solution, Bi(2)(Hsal)(6).Co(acac)(3) eliminates Bi(Hsal)(3) to give the 1:1 adduct Bi(Hsal)(3).Co(acac)(3). The 2:1 heterobimetallic molecular compounds undergo facile thermal decomposition on heating in air to 475 degrees C to produce heterometallic oxide materials, which upon annealing for 2 h at 700 degrees C form crystalline oxide materials. The synthetic approach detailed here represents a unique, general approach to the formation of heterobimetallic bismuth-based coordination complexes via the coordination of M(acac)(3) complexes to bismuth(III) salicylate.     

A Ditopic Catechol Phosphane as Building Block for Selective Construction of Heterometallic Complexes with Early and Late Transition Metal Centers

Base‐assisted reaction of catechol phosphane 2 (H₂L) with [M′Cl₂(cod)] (cod = 1, 5‐cyclooctadiene, M′ = Pd, Pt) yielded chelate complexes [M′(HL)₂] ( 7a, b ). Spectroscopic and single‐crystal X‐ray diffraction studies revealed that both complexes feature cis‐configuration of the P‐ and O‐donor atoms in solution and in the solid state. Reaction of 7a, b with acetylacetonato or alkoxide complexes [MO₂(acac)₂] (M = Mo, W), [VO(acac)₂], [{Ti(μ‐O)(acac)₂}₂], or Ti(OiPr)₄ gave good to excellent yields of early‐late heterometallic complexes [MO_n(μ‐L)₂M′] (MO_n = MoO₂, WO₂, VO; 8a, b – 10a, b ) or [Ti(RO‐1κO)₂(μ‐L ‐1κ²O, O'‐2κ²P, O)₂Pd] (R = Me, iPr; 11a, b ), which were inaccessible via other synthetic routes. Spectroscopic and single‐crystal X‐ray diffraction studies revealed that the early metal centres in 8a, b, 9b and in 11b feature distorted octahedral coordination spheres with rigid transoid alignment of the catechol ring planes. Vanadium complexes 10a, b exhibit a square‐pyramidal coordination sphere with cisoid alignment of the catechol ring planes and evidence for intermolecular pairing via weak VO ··· Pd contacts in the solid state; complexes 8 , 9 do not undergo conformational inversion on the NMR time‐scale. The molecular structure of Ti complex 11a is characterized by a different orientation of the catechol moieties, which can be envisaged to picture an intermediate state during a configuration inversion process, and a strong hydrogen bridge between a terminally coordinated catecholato‐oxygen atom and a solvent molecule (MeOH). Solution NMR studies indicate that the (MeO)₂Ti(μ‐L)₂M' framework is in this case conformationally labile and that the MeO ligands undergo intermolecular dynamic exchange with the solvent.     

Features of intermolecular interactions in the crystals of metal acetylacetonates

For 12 acetylacetonates of the composition M(acac) _n (n = 2, 3, or 4) and M(acac)(C₂H₄)₂ (M is a metal) the total area (⁰ S) of the faces of Voronoi-Dirichlet polyhedra (VDP) corresponding to all intermolecular contacts of one molecule in the crystal structure and the total volume of pyramids (⁰ V), whose bases are formed of such faces and the vertices are occupied by the nuclei of atoms participating in intermolecular contacts, are determined. The key features of non-bonded interactions are considered. The existence of a linear dependence of the sublimation enthalpy of acetylacetonates on the ⁰ S or ⁰ V parameters of their molecular VDP is revealed. It is shown that the sublimation enthalpy of Ga(acac)₃ requires the refinement and theoretically should be 124 kJ/mol.     

5-(Pyridylmethylidene)-substituted 2-thiohydantoins and their complexes with CuII, NiII, and CoII: Synthesis, electrochemical study, and adsorption on the cystamine-modified gold surface

A series of Cu^II, Ni^II, and Co^II complexes with 5-(pyridylmethylidene)-substituted 2-thiohydantoins (L) were synthesized by the reactions of the corresponding organic ligands with MCl₂·nH₂O. The resulting complexes have the composition LMCl₂ (M = Cu or Ni) or L₂MCl₂ (M = Co). The reactions with N(3)-unsubstituted thiohydantoins afford complexes containing four-membered metallacycles, in which the metal ion is coordinated by the S and N(3) atoms of the thiohydantoin ligand. The reactions of N(3)-substituted thiohydantoins give complexes in which the S and N(1) atoms are involved in coordination. Study by IR spectroscopy demonstrated that the pyridine nitrogen atom is not involved in coordination. Based on the results of electrochemical study of the ligands and complexes by cyclic voltammetry and calculation of their frontier orbitals by the PM3(tm) method, the mechanism of oxidation and reduction of these compounds was proposed. In the first reduction and oxidation steps, the metal atom in the copper and nickel complexes remains, apparently, intact, and these processes occur with the involvement of the ligand fragments, viz., the coordinated thiohydantoin ligand and chloride anion, respectively. In the cobalt complexes, the first reduction step occurs at the ligand; the first oxidation state, at the metal atom. Measurements of the contact angle of aqueous wetting and electrochemical study demonstrated that carboxy-containing 2-thiohydantoins and their complexes can be adsorbed on the cystamine-modified gold surface. The structures of the complexes on the surface differ from the structures of these complexes in solution. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 978–990, June, 2006.     

Solvent effects in gel chromatography: the relationship between the distribution coefficients of metal acetylacetonates and solubility parameters of solvents

Distribution coefficients of metal acetylacetonates such as Al(acac)₃, Cr(acac)₃ and Co(acac)₃ etc., and n-alkanes in porous polystyrene gel chromatography with ten organic solvents as eluants were estimated and compared with those obtained by the batch method in two solvent systems. Solubility parameters of the metal acetylacetonates were determined. There is a relationship between the distribution coefficients and solubility parameters of solutes and solvents. Two factors (steric exclusion and partition) play important rôles in each solvent for the separation mechanisms of the present systems. The latter factor was estimated successfully from a regular solutions approximation.     

Transition‐Metal Chemistry of Alkaline‐Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr,Ba)

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)₃ (M=Sr, Ba) in low‐temperature Ne matrix. Both complexes are characterized by a D₃ symmetric structure involving three equivalent η⁶‐bound benzene ligands and a closed‐shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal–ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n?1)d AOs of M and strong backdonation from the occupied (n?1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20‐electron complexes have 18 effective valence electrons, and, thus, fulfill the 18‐electron rule if only the metal–ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.