Energetics of the first and second coordination spheres of transition metal ions

For complexes of transition metals (manganese, iron, cobalt, nickel) with monodentate ligands, equilibrium metal-ligand distances and ligand bond energies in the first and second coordination spheres have been calculated by the CNDO method. Some effects of ligand bond energies in different coordination spheres are analyzed. These effects significantly differ between the first and second coordination spheres. In the first sphere, the ligand bond energy is mainly determined by the nature of the central ion and the type of donor atom of the ligand, but weakly depends on the structure of the ligand. Conversely, in the second coordination sphere, the ligand bond energy weakly depends on the nature of the central ion and the type of donor atom, but considerably depends on the structure of the ligands in the first coordination sphere. In the second coordination sphere, ligand binding is determined by ligand interactions with both the central ion and the ligands of the first sphere. In the general case, when strong specific interactions between ligands are absent, the energetics of the second sphere is determined by the size of the inner-spheric ligands, which may be considered to be a specific steric effect. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 370–374, March–April, 1995. Translated from L. Smolina     

Ligand displacement reactions of dimer and trimer pyridyl ligand/transition metal complexes

Ligand exchange reactions of pyridyl ligand/transition metal complexes are examined in a quadrupole ion trap mass spectrometer to evaluate the ability of multidentate ligands to displace other pyridyl ligands in complexes where the charge is highly delocalized and there is a great degree of ligand repulsions. Partially or fully coordinated transition metal ions in dimer or trimer species involving small mono- or bidentate pyridyl ligands undergo ligand displacement reactions with larger bi- and tridentate pyridyl ligands. Larger ligands with greater chelation abilities, such as 1,10-phenanthroline and 2,2′:6,2″-terpyridine, are often able to simultaneously displace two nonchelating ligands from a partially coordinated metal ion. However, the analogous reactions involving displacement of bidentate chelating ligands from more fully coordinated transition metal ion complexes are nearly quenched. In other cases, mixed-ligand dimer and trimer complexes are observed, indicating step-wise displacement of the initially complexed ligands.     

THE INFLUENCE OF THE LIGANDS ON THE LUMINESCENCE OF METAL IONS*

The nature of the ligands determines the luminescence properties of the central metal ion. This is illustrated on the following examples: (i) nonradiative transitions in the strong coupling model; (ii) nonradiative transitions in the weak coupling model; (iii) energy transfer from ligand to central ion. Several applications are dealt with. The cryptand ligands appear to play a special role.     

Synthesis and Characterization of Some Mixed Ligand-Selenito and Tellurito Chromium(III) Complexes

New mixed ligand Cr(III) complexes were prepared where diamine or oxalato ligands are coordinated together with either tellurito, selenito, or hydrogenselenito ions to form nine octahedral complexes. The complexes were characterized by chemical analyses, IR and UV-visible spectra, magnetic, and conductivity measurements. The tellurito and selenito ligands act as monodentate ligands, coupled with the bidentate diamine ligands. On the other hand, they act as bidentate chelate ligands when coordinated together with the oxalate ligand. However, hydrogenselenite ion act as a monodentate ligand coupled with the oxalate ligand. IR spectra indicated that the inorganic ligands are coordinated to the Cr(III) ion through their oxygen atoms. One of the bulky diamine molecules, 1,2-pn or 1,3-pn, was freed from the coordination sphere of Cr(III) on the addition of the bulky inorganic anions and was replaced by two water molecules.     

堆积效应与弱共价键分子结构的模拟

<正> 在有限的空间中排列物体时,被排列物体的几何因素对排列方式会产生影响。我们将这种效应称为“堆积效应”。用积木游戏可形象说明堆积效应。如在一个圆形积木盒中放进四块圆积木(图1),显然这些积木不能任意排列。积木B只能分别置于积木A的两侧。它们“摄动”的范围是很小的。同样,在配位化合物的分子结构中,配位空间只有4π立体角,     

Calculation of the equilibrium constants and cooperative parameters of formation of Cu(II) chloride complexes in nonaqueous solvents

Copper(II) distributions over chloride complexes in various organic solvents were analyzed in terms of a modified matrix model. The equilibrium coordination constants of a first ligand and the corrections for the mutual influence between the ligands during the complexation were calculated. It was demonstrated that displacement of the solvent molecule by a chloride ion from the inner coordination sphere of the Cu(II) ion is always of anticooperative character. In MeCN, addition of a chloride ion to a copper ion follows the simplest additive scheme of coordination of the ligand with equivalent coordination vacancies. Possible reasons for nonadditive complexation in DMF, DMSO, trimethyl phosphate, and propylene carbonate are discussed.     

Assembly of hydrophobic shells and shields around lanthanides

Luminescent lanthanide complexes have been developed, based on the assembly of bulky ligands around the lanthanide ion, to provide shell-type protection of the ion from coordinated solvent molecules. Aryl-functionalised imidodiphosphinate ligands (tpip and Metpip) provide a bidentate anionic site that leads to hexa-coordinate lanthanide complexes in which the aryl groups surround the ion. There are twelve phenyl groups around the lanthanide that act as "remote" (from the binding site) sensitisers for the metal ion. It is shown that these ligands are suitable for sensitising luminescence for all the lanthanides that emit in the visible range, namely, SmIII, EuIII, TbIII, DyIII. A "builtin" shield on the ligand is designed to provide a complete block of the approach of water to the lanthanide ion. The synthesis of the ligands and their lanthanides complexes as well as detailed photophysical studies of the complexes in solution and in the solid-state are presented.     

Effect of acetate and EDTA ligands on Hydrolysis of the Iron(III) Ion in sodium chloride medium

The hydrolysis of the iron(III) ion in sodium chloride medium without organic ligands and in the presence of acetate and EDTA ligands was studied by emf method, at 25°C. The data indicate the effect of the organic ligands. In the presence of acetate ion the beginning of hydrolysis of the iron(III) ion is slightly shifted toward lower pH values, while in the presence of EDTA, as a strong complex forming ligand, the beginning of hydrolysis is shifted toward higher pH values for 2.5 pH units.     

Binding of gaseous Fe(III)-heme cation to model biological molecules: Direct association and ligand transfer reactions

The binding of a variety of ligands with Fe(III)-heme(+) ion, prosthetic group of heme proteins, has been studied in the gas phase by ESI-FT-ICR mass spectrometry. The ligands have been selected among substrate molecules of heme proteins (e.g., NO, nitroso compounds) or among model compounds acting for the functional groups that are present in the protein backbone (e.g., amines, thioethers, nitriles, ketones, amides, etc.). Both the kinetic and the thermodynamic features of the addition reactions are reported. Fe(III)-heme(+) ions react faster with lone pair donor ligands as the reaction becomes increasingly thermodynamically favored (higher heme cation basicity of the ligand, HCB, namely -DeltaG degrees for the ligand addition reaction). In turn HCBs correlate in general with the gas phase basicity toward the proton of the various ligands. A ligand addition equilibrium is established with weaker ligands, methanol, acetonitrile and acetone, yielding absolute HCB values, whereas ligand transfer equilibriums allowed a scale of relative (and absolute) HCBs to be constructed. NO displays exceptional binding properties towards Fe(III)-heme(+), unrelated to the low gas phase basicity toward the proton of this molecule, which is clearly the basis for the paramount role of heme proteins in NO binding and regulation.     

Electrostatic bonding models: A test on group 1 and 2 metal complexes with H2O,NH3, H2S,PH3, and related ligands

Electrostatic models frequently proposed to describe ion–molecule interactions have been tested on the adducts formed by Group 1 and 2 cations with H₂O, NH₃, H₂S, PH₃, their methyl analogs, and their anions. The results from the model calculations were compared with all-electron calculations (geometry optimized, MP2, TZP basis sets) carried out on adducts formed with Li⁺, Na⁺, K⁺, Ca²⁺, and Mg²⁺. The electrostatic potential model was utilized in two ways: The attraction of the point charge was calculated with and without relaxation of the ligand. A third model allowed relaxation of the ligand but treated the cation as a frozen core. The final model was the crude point charge/point dipole approximation. At long range, the models satisfactorily track the effects on energy of gross changes in the ion–ligand interaction (monovalent versus divalent ions, neutral ligands versus anions, parent ligands versus methyl derivatives), but correlation at close range is poor, especially for binding by divalent cations. The hypothesis that the calculated strength of cation–dipole binding is dependent on calculated dipole moment could not be verified. © 1995 by John Wiley & Sons, Inc.     

Ab initio study of intriguing coordination complexes: a metal field theory picture

Two noninnocent ligands are theoretically studied using wave function based methods to demonstrate their ability to undergo singlet-triplet transition under the effect of an external charge mimicking the electrostatic role of a metal ion. It is shown that the singlet-triplet energy difference is very sensitive to the metal ion charge which tunes the HOMO-LUMO energy difference of these ligands. While the latter is reduced as the charge is enhanced in the glyoxal-bis-(2-mercaptoanil) (gma) ligand, it is increased in the bis(imino)pyridine diradical ligand. This result shows a strong analogy with the crystal field theory, interchanging the roles played by the metal ion and the ligand. As the metal ion is explicitly treated in the Fe(gma)CN complex, this analogy can be pushed further resulting in a "metal field theory" conceptualization.     

Investigation of uranyl nitrate ion pairs complexed with amide ligands using electrospray ionization ion trap mass spectrometry and density functional theory

Ion populations formed from electrospray of uranyl nitrate solutions containing different amides vary depending on ligand nucleophilicity and steric crowding at the metal center. The most abundant species were ion pair complexes having the general formula [UO(2)(NO(3))(amide)(n=2,3)](+); however, singly charged complexes containing the amide conjugate base and reduced uranyl UO(2)(+) were also formed as were several doubly charged species. The formamide experiment produced the greatest diversity of species resulting from weaker amide binding, leading to dissociation and subsequent solvent coordination or metal reduction. Experiments using methyl formamide, dimethyl formamide, acetamide, and methyl acetamide produced ion pair and doubly charged complexes that were more abundant and less abundant complexes containing solvent or reduced uranyl. This pattern is reversed in the dimethylacetamide experiment, which displayed lower abundance doubly charged complexes, but augmented reduced uranyl complexes. DFT investigations of the tris-amide ion pair complexes showed that interligand repulsion distorts the amide ligands out of the uranyl equatorial plane and that complex stabilities do not increase with increasing amide nucleophilicity. Elimination of an amide ligand largely relieves the interligand repulsion, and the remaining amide ligands become closely aligned with the equatorial plane in the structures of the bis-amide ligands. The studies show that the phenomenological distribution of coordination complexes in a metal-ligand electrospray experiment is a function of both ligand nucleophilicity and interligand repulsion and that the latter factor begins exerting influence even in the case of relatively small ligands like the substituted methyl-formamide and methyl-acetamide ligands.     

混配络合物中配位体最佳浓度比的理论研究

我们认为,如金属离子M能与配位体A和B形成二元络合物,那么在配位数允许和不存在空间障碍的情况下,总有三元络合物形成.但由于两种配位体之间的浓度比例调节不当,会造成三元络合物在溶液里的所有组份中占很小的比例,这样,往往给人们一个错觉,即没有三元络合物形成,三元混配络合物在溶液里所有组份中所占的比例达到极大时,两配位体之间的浓度比例是可以通过计算得到的。     

Spectrophotometric determination of equilibrium constants of two mutual competitive reactions by the method of isosbestic points

We have developed a method that makes use of dual isosbestic points in a dual ligand, single metal system that allows the determination of the equilibrium constants of complexes in which two ligands compete for the same metal ion, and one complex is colourless. The competition of methyl thymol blue and citrate was used to test the model.     

The coordination of the catalytic zinc ion in alcohol dehydrogenase studied by combined quantum-chemical and molecular mechanics calculations

Summary The coordination number of the catalytic zinc ion in alcohol dehydrogenase has been studied by integrated ab initio quantum-chemical and molecular mechanics geometry optimisations involving the whole enzyme. A four-coordinate active-site zinc ion is 100–200 kJ/mol more stable than a five-coordinate one, depending on the ligands. The only stable binding site for a fifth ligand at the zinc ion is opposite to the normal substrate site, in a small cavity buried behind the zinc ion. The zinc coordination sphere has to be strongly distorted to accommodate a ligand in this site, and the ligand makes awkward contacts with surrounding atoms. Thus, the results do not support proposals attributing an important role to five-coordinate zinc complexes in the catalytic mechanism of alcohol dehydrogenase. The present approach makes it possible also to quantify the strain induced by the enzyme onto the zinc ion and its ligands; it amounts to 42–87 kJ/mol for four-coordinate active-site zinc ion complexes and 131–172 kJ/mol for five-coordinate ones. The four-coordinate structure with a water molecule bound to the zinc ion is about 20 kJ/mol less strained than the corresponding structure with a hydroxide ion, indicating that the enzyme does not speed up the reaction by forcing the zinc coordination sphere into a structure similar to the reaction intermediates.     

基于2,6-二(4-羧基苯亚甲基)环己酮的金属-有机框架化合物的合成与表征

以2,6-二(4-羧基苯亚甲基)环己酮(H2L)为配体得到一例锰金属-有机框架化合物[MnL]n,并运用红外、热重、循环伏安、固体紫外、X射线光电子能谱和X射线单晶衍射对其进行表征.单晶衍射分析表明该配合物属于三斜晶系,空间群P(1),不对称单元由Mn(Ⅱ)离子和一个L2-配体组成.配体两端的羧基均为单齿配位,配体中间...     

亲和毛细管电泳间接紫外检测法测定金属络合物的亲和常数

 采用亲和毛细管电泳间接紫外检测方法 ,根据“峰漂移”模型 ,通过迁移时间的测定 ,可以获得在水体系中有极低亲和常数的金属络合物的亲和常数。将该方法分别应用于镁离子 柠檬酸体系和锰离子 酒石酸体系 ,在 pH为 5 .0 1,运行电压为 2 0kV ,缓冲溶液组成为咪唑和醋酸的条件下 ,测定了缓冲溶液中加入不同浓度配体后金属离子迁移时间的变化 ,经过数据处理后得出它们的亲和常数对数值分别是 3.2 7和 2 .2 8,与文献值较为一致。该方法适用于结合比为 1∶1的金属络合物的亲和常数的测定。     

Mass spectrometric studies on small open-chain piperazine-containing ligands and their transition metal complexes

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry was used to characterize the complexes formed between open-chain piperazine-containing ligands and transition metal salts (Cobalt, Copper, Zinc, and Cadmium as chlorides, nitrates, and acetates). Only single-charged complexes were observed, formed of one ligand (L) and mainly one metal ion (M). Since the net charge of the complexes was one, a counterion (X) was attached to some of the complexes, with formation of [L + M + X]+ complexes, and a proton was lost from others, as in [L - H + M]+ complexes. In most cases the composition of the complexes was more dependent on the ligand than the metal salt. Collision-induced dissociation measurements showed that complexes with related composition often differed in structure, or that interactions between the ligand and the metal ion were not alike. The metal ion influenced considerably the fragmentation pathways of the ligands, so that the fragmentation products could be used to deduce the binding sites of the metal. The variations observed in fragmentation behavior of complexes possessing the same ligand but different metal ions can mostly be explained by the ionic radius and electronic configuration of the metal ion. The results indicated a preference of the piperazine ring of the coordinated ligand for the boat conformation.     

Impact of ligand protonation on eigen-type metal complexation kinetics in aqueous systems

The impact of ligand protonation on metal speciation dynamics is quantitatively described. Starting from the usual situation for metal complex formation reactions in aqueous systems, i.e., exchange of water for the ligand in the inner coordination sphere as the rate-determining step (Eigen mechanism), expressions are derived for the lability of metal complexes with protonated and unprotonated ligand species being involved in formation of the precursor outer-sphere complex. A differentiated approach is developed whereby the contributions from all outer-sphere complexes are included in the rate of complex formation, to an extent weighted by their respective stabilities. The stability of the ion pair type outer-sphere complex is given particular attention, especially for the case of multidentate ligands containing several charged sites. It turns out that in such cases, the effective ligand charge can be considerably different from the formal charge. The lability of Cd(II) complexes with 1,2-diaminoethane-N,N'-diethanoic acid at a microelectrode is reasonably well predicted by the new approach.     

The geometry of nonmetal hydrides and the ligand radius of hydrogen

The aim of this paper was to investigate why the geometries of nonmetal hydrides are often not in accordance with the VSEPR model. From a consideration of interligand distances in a variety of BX(4), CX(4), and NX(4) molecules where X is a ligand or a lone pair and in which there are at least two H ligands we have shown that the hydrogen ligands are essentially close-packed. For each of the central atoms we have obtained a value for the ligand radius of hydrogen. These radii decrease with decreasing negative charge and increasing positive charge of the hydrogen ligand as the electronegativity of the central atom increases, as has been found previously for other ligands such as F and Cl. We show that ligand-ligand intractions are an important factor in determining bond angles in hydrides and that the ligand close-packing (LCP) model gives a better explanation of bond angles than the VSEPR model according to which bond angles depend on the electronegativity of the ligand rather than on its size. For example, although the very small angles in PH(3) and SH(2) are not in accord with the VSEPR model, they are consistent with the LCP model in that they are a consequence of the small size of hydrogen ligands which are pushed together by the lone pairs until they are almost close-packed.