Bond valence sums in coordination chemistry. The calculation of the oxidation state of samarium in complexes containing samarium bonded only to oxygen

A simple method is presented for calculating the oxidation state of Sm in complexes where Sm is bonded only to O ligands. A total of 88 SmO(n)() fragments with n = 4-12 were retrieved from the Cambridge Structural Database and were analyzed using the bond valence sum (BVS) method. New R(0) values for Sm(II)-O of 2.116(21) A and for Sm(III)-O of 2.055(13) A were derived. The average R(0) value of 2.086 A gives a good approximation of the oxidation state of the Sm ion, either +2 or +3, from the observed distances without any assumptions. The Sm-O distances for +2 and +3 complexes with coordination numbers of 4-11 are tabulated and reflect the requirement that the BVS must equal the oxidation state. The distances for CN = 12 were not included because of problems with the reported crystal structures. Several X-ray structure determinations where the BVS and the oxidation state did not agree are discussed.     

Bond valence sums in coordination chemistry. Calculation of the oxidation state of chromium in complexes containing only Cr-O bonds and a redetermination of the crystal structure of potassium tetra(peroxo)chromate(V)

A simple method for calculating the oxidation state of Cr in complexes containing only Cr-O bonds is presented. A total of 242 CrOn fragments with n = 3-6 were retrieved from the Cambridge Structural Database (CSD) and, together with the data for K3CrO8, were analyzed using the bond valence sum method. New R0 values for Cr(II) of 1.739(21) A, Cr(III) of 1.708(7) A, Cr(V) of 1.762(14) A, and Cr(VI) of 1.793(7) A were derived. An average R0 value of 1.724 A for Cr-O reproduces the oxidation state of 96 of the 110 Cr(II), Cr(III), and Cr(IV) CrOn complexes (n = 3-6) and that of K3CrO8 within 0.30 valence units. The crystal structure of K3CrO8 was redetermined at 173 K to provide accurate data for a Cr complex with both high oxidation state and coordination number. Potassium tetraperoxochromate(V), K3CrO8, is tetragonal, Space group I42m, a = b = 6.6940(3) A, c = 7.7536(5) A, Z = 2. The difficulties with fitting the observed valence for Cr(V) and Cr(VI) complexes with coordination numbers 4 and 5 are discussed. The use of bond valence sums in gaining chemical insight into Cr complexes with noninnocent ligands and in establishing oxidation states in Cr clusters is presented. An analysis of the Cr-O bond distances used in the calculations shows a large range of values that can be understood in terms of the bond valence sum calculation.     

Versatile coordination chemistry of rhodium complexes containing the bis(trimethylsilylmethyl)tellane ligand

When RhCl₃ · 3H₂O was treated with an excess of Te(CH₂SiMe₃)_2, a mononuclear mer-[RhCl₃{Te(CH₂SiMe₃)₂}₃] (1) was observed as the main product. By reducing the metal-to-ligand molar ratio, dinuclear [Rh₂(μ-Cl)₂Cl₄{Te(CH₂SiMe₃)₂}₄] (2) was obtained in addition to 1. Further reduction of the metal-to-ligand ratio resulted in the formation of [Rh₂(μ-Cl)₂Cl₄(OHCH₂CH₃){Te(CH₂SiMe₃)₂}₃] (3). The treatment of mer-[RhCl₃(SMePh)₃] (4) with two equivalents of Te(CH₂SiMe₃)₂ affords a mixture of mer-[RhCl₃{Te(CH₂SiMe₃)₂}₃] (1) and mer-[RhCl₃{Te(CH₂SiMe₃)₂}₂(SMePh)] (5). All complexes 1-4 and 5 · ½EtOH were characterized by X-ray crystallography and ¹²⁵Te NMR spectroscopy, where appropriate. The definite assignment of the ¹²⁵Te chemical shifts enabled a plausible discussion of the assignment of some unknown resonances that were observed in the NMR spectra.     

Thermally induced changes in the oxidation state of cerium(IV)

Cesium hexachlorocerate(IV), Cs₂CeCl₆ (I) and sodium pentakis(carbonato)cerate(IV), Na₆Ce(CO₃)₅·12H₂O (II) have been investigated in air by simultaneous TG/DTA, FTIR and XRD in order to follow the oxidation state of cerium during their thermal treatment. The thermal decomposition of the hexachloro compound (I) is accompanied by a double change in the oxidation state of cerium. First, in an inner reduction-oxidation reaction, chlorine is evolved and a Cs₂CeCl₅ phase is obtained. The immediately starting oxidation of this Ce(III) species caused various phase transitions in the CeCl₃-CsCl system formed. The presence of Cs₃CeCl₆ above 400°C can also be assumed and finally this phase also oxidizes into CeO₂ with the formation of CsCl as by-product. In the case of the pentacarbonato complex (II), no Ce(III) species were detected. The final products of its decomposition were CeO₂ and Na₂CO₃.     

The coordination chemistry of macrocyclic diamides. Transition metal complexes of 5,7-dioxo-1,4,8,11-tetraazacyclotetradecane

Summary A convenient preparation of the 14-membered macrocyclic diamide 5,7-dioxo-1,4,8,11-tetraazacyclotetradecane (LH₂) is described. The pK _NH ⁺ values of the ligand are pK₁ = 5.76 and pK₂ = 9.63 at 25° and I = 0.1 mol dm^–3 (KNO₃). With metal ions able to ionise amide hydrogens, the ligand acts as a planar quadridentate, L^2–. Thus copper(II) and nickel(II) give the neutral complexes ML, and conductivity measurements confirm that they are nonelectrolytes in aqueous solution. Both the nickel(II) and copper(II) complexes are acid labile unlike the analogues of 1,4,8,11-tetraazacyclotetradecane (cyclam).The cobalt(III) complex [CoL(NH₃)₂]Cl has been characterised and¹H n.m.r. measurements established the N-meso stereochemistry at the chiral nitrogen centres.     

An application of second-order n-electron valence state perturbation theory to the calculation of excited states

n–electron valence state perturbation theory (NEVPT) is a form of multireference perturbation theory where all the zero-order wave functions are of multireference nature, being generated as eigenfunctions of a two–electron model Hamiltonian. The absence of intruder states makes NEVPT an interesting choice for the calculation of electronically excited states. Test calculations have been performed on several valence and Rydberg transitions for the formaldehyde and acetone molecules; the results are in good accordance with the best calculations and with the existing experimental data.Contribution to the Jacopo Tomasi Honorary Issue     

Advances in the coordination chemistry of nitrogen ligand complexes of coinage metals

Coinage metals nitrogen chemistry has not been studied extensively until recently. The focus of this review is the base- and halide-free complexes of the monoanionic nitrogen ligands. This review describes how minor ligand modifications can result in a drastic change in the metal–metal interactions in multinuclear compounds. Crystal structures of these complexes show individual complexes, dimers, supramolecular columnar packing or more complex supramolecular aggregates. Bulky substituents on the ligands can prevent intermolecular metal–metal interactions or the formation of supramolecular architectures. The nuclearity and metal–metal interactions in these complexes are controlled by ligand steric and electronic factors and solvent of crystallization. Many classes of nitrogen ligand coordination compounds have given rise to advances in several fundamental and applied research aspects. Recent potential applications of nitrogen ligand complexes are highlighted particularly for those complexes included in this review.     

Bond energy of surface oxygen in multicomponent oxide catalysts for propylene oxidation to acrolein

Bond energy of surface oxygen for the multicomponent oxide catalyst Mo₁₂Bi₁Ni_2.5Fe₃Co_4.5K_0.07P₁/SiO₂ has been measured in conditions of propylene oxidation and found to be 272 kJ/mol.

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Mo₁₂Bi₁Ni_2,5Fe₃Co_4,5K_0.07P₁/SiO₂, , 272 /.

     

Exploratory studies on coordination chemistry of a redox-active bridging ligand: synthesis, properties and solid state structures of the complexes

The explorative coordination chemistry of the bridging ligand TTF-PPB is presented. Its strong binding ability to Co(II) and then to Ni(II) or Cu(II) in the presence of hexafluoroacetylacetonate (hfac(-)), forming new mono- and dinuclear complexes 1-3, is described. X-ray crystallographic studies have been conducted in the case of the free ligand TTF-PPB as well as its complexes [Co(TTF-PPB)(hfac)(2)] (1) and [Co(hfac)(2)(μ-TTF-PPB)Ni(hfac)(2)] (2). Each metal ion is bonded to two bidentate hfac(-) anions through their oxygen atoms and two nitrogen atoms of the PPB moiety with a distorted octahedral coordination geometry. Specifically, nitrogen donor atoms of TTF-PPB adopt a cis-coordination but not in the equatorial plane, which is quite rare. Electronic absorption, photoinduced intraligand charge transfer ((1)ILCT), and electrochemical behaviour of 1-3 have been investigated. UV-Vis spectroscopy shows very strong bands in the UV region consistent with ligand centred π-π* transitions and an intense broad band in the visible region corresponding to a spin-allowed π-π* (1)ILCT transition. Upon coordination, the (1)ILCT band is bathochromically shifted by 3100, 6100 and 5900 cm(-1) on going from 1 to 3. The electrochemical studies reveal that all of them undergo two reversible oxidation and one reversible reduction processes, ascribed to the successive oxidations of the TTF moiety and the reduction of the PPB unit, respectively.     

Extending the chemistry of the uranyl ion: Lewis acid coordination to a U=O oxygen

Treatment of the thf adduct UO2(NCN)thf (NCN = [(Me3SiN)CPh(NSiMe3)]) (1) with 2 equiv of B(C6F5)3 provides UO{OB(C6F5)3}(NCN)2 (2) the first example of a neutral uranyl complex exhibiting Lewis basic behavior. The crystal structure of 2 shows a U=O-B interaction with an elongated U=O bond (1.898(3) A). Raman spectroscopy suggests weakening of the O=U=O bonding, giving the lowest reported symmetric stretching frequency for a monomeric uranyl complex, nu1 = 780 cm-1. The borane can be selectively removed using PMe3 to give the coordinatively unsaturated UO2(NCN)2 (3) or using tBuNC to provide UO2(CNBut)(NCN)2 (4), the first example of an isonitrile coordinated to uranium.     

The kinetics of silver oxidation with oxygen in solutions containing thiocyanate-thiourea and thiocyanate-thiosulfate mixtures

The kinetic parameters of the heterophase processes were determined by the rotating disk equiaccessible surface method. The mechanism of the rate-determining step was shown to depend on the molar ratio between reagent concentrations. The ratios at which the highest reaction rates were observed can be used to prepare combined solvents for extracting silver and gold from natural and secondary raw materials.     

Application of139La NMR spectroscopy to the coordination chemistry of lanthanum. Ligand exchange in cyclopentadienyl and metal carbonyl complexes of lanthanum

The ligand-exchange equilibria in cyclopentadienyl and metal carbonyl complexes of lanthanum have been qualitatively studied by¹³⁹La NMR spectroscopy: 2X¹X²X³La X¹X² ₂ La + X¹X³, La, where X¹=X²=Cp, X³=I; X¹=X² = 1, 3-(Me₃Si)₂C₅H₃, X³=Cl; X¹=Ru(CO)₂Cp; X²=Cp, X³=I. Similar equilibria are typical of the coordination chemistry of light lanthanides.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 578–581, March, 1993.     

Structural diversity in coordination chemistry of tridentate and tetradentate polyphosphines of Group 6 to 10 transition metal complexes

Tridentate and tetradentate polyphosphines offer a huge variety of coordination modes to transition metals which lead, depending on the metal, to very different structural features in the resulting complexes. Steric effects being crucial in metal–phosphine complexes reactivity, a good knowledge of the molecular structures of the species is required both in the solid state and in solution. This article reviews from a structural point of view the monometallic and symmetrical homobimetallic complexes of the transition elements of Group 6 to 10 with tridentate and tetradentate phosphines. Concerning the classical triphosphines and tetraphosphines, emphasis was put on advances reported after the year 1994, since comprehensive reviews have covered the former period. Several anterior relevant results are, however, briefly mentioned when necessary. A second part is devoted to nitrogen- and sulfur-containing derivatives potentially tridentate and tetradentate ligands, and their coordination to the above-mentioned metals. The last part describes the complexes obtained with the less classical ferrocenyl polyphosphine ligands or their nitrogen-containing derivatives: each ligand having a potential tridentate or tetradentate coordination from either phosphorus or nitrogen donor atoms. The literature cutoff date was during the second half of 2000, but in a few cases, references to important work appearing during 2001 were made; however such coverage should be completed in a future compilation. An exhaustive quoting of catalytic applications and reaction chemistry was beyond the scope of this article mainly devoted to structural works. Nevertheless, in order to illustrate the importance of this chemistry, efforts were made to provide the reader with recent references that have marked the field, even in the absence of X-ray structural characterization.     

Direct calculation of lattice sums. A method to account for the crystal field effects

A method of direct calculation of lattice sums in three-dimensional crystals is reported. The method is based on annihilation of some lowest multipole moments of the unit cell by a redefinition of the unit cell content. As a result, properties of the infinite crystal can be calculated as usual by taking a finite cluster of unit cells, but surrounded by an additional surface layer of a charge density (e.g., a layer of point charges). This charge density distribution produces the electric field approximating that one of the rest of the infinite crystal. The method proposed is easily applicable in the SCFLCAO procedure as well as in any method using a cluster representation for an infinite crystal. The validity of the infinite crystal model for a finite crystal is also discussed.