High-spin complexes


High spin octahedral iron(III) compounds are and have a ground state. AH excited states have a different spin multiphcity. Consequently, d—d transitions are spin and parity forbidden and many simple salts and complexes have Httle or no color. These bands are often obscured by the low energy tails into the visible of ligand-to-metal charge-transfer absorptions in the near-uv region. When strong color occurs, charge-transfer absorptions are usually responsible. The magnetic moments of high spin complexes are all very close to the spin-only value of 5.47 x 10 J/T (5.9 //g) because of the absence of an orbital angular momentum contribution or coupling to excited states. Deviations from this value are often the result of antiferromagnetic interactions between two or more iron centers. Low spin complexes typically have magnetic moments of 2.04 x 10 J/T (2.2 //g ). In lower symmetry ligand fields such as square pyramidal, intermediate spin states with moments of 3.71 x 10 J/T (4.0 //g ) are possible.  [c.433]

Siderophores. Iron is not readily available at physiological pH because it is present as the insoluble hydrated iron(III) oxide, which has 10 . Bacteria synthesize chelating agents to faciUtate the solubilization of iron from the environment, transport into the organism, and release of iron. Most contain negatively charged oxygen-donor groups which preferentially complex iron(III) and afford octahedral, high spin complexes called siderophores (21,22). The two principal classes of donor groups employed are catecholates (14) and hydroxamates (15).  [c.443]

Write down the crystal held orbital configurations of the following transition metal complexes [Cu(H20)6], [V(H20)6], [Mn(H20)6] -high spin, [Co(NH3)6] +-low spin. Explain why [Mn(H20)6] is colourless.  [c.288]

The iron(II) free-ion ground state is spHt by octahedral fields or tetrahedral fields into and states. Tetrahedral and high spin octahedral complexes have magnetic moments of about 4.6-4.8 x 10 J/T (5.0—5.2 Bohr magnetons). The octahedral complexes exhibit a single weak d—d transition, which falls in the visible/near ir region of the spectmm and is broadened by the J ah n-Teller effect. Strong field ligands can cause spin pairing to afford diamagnetic low spin octahedral complexes that have a M ground state. These may be intensely purple or red in color, owing to metal-to-ligand charge-transfer bands. Low spin octahedral iron(II) complexes are kineticaHy inert, as are the isoelectronic low spin cobalt(III) complexes. Examples of high, low, and intermediate spin S = 1) complexes are known for square planar complexes.  [c.433]

Although high spin iron(III) complexes are usually kinetically labile, siderophores are inert and can be resolved into individual optical isomers. An important issue, then, is how the microorganism releases iron from the siderophore. The redox potential of iron-enterobactin is probably too negative for reduction to iron(II), which would be more labile, to occur under physiological conditions. Suggestions include cleavage of the siderophore backbone and protonation of the chelate groups.  [c.443]

However, high spin (P and (P species, which possess 4, 5, and 4 unpaired electrons, respectively, are labile, as are (P through (P octahedral complexes. In addition to the inert (P systems, low spin (P and (P complexes are inert to rapid substitution. The (P species are the least labile of the configurations classed as labile.  [c.170]

Aqueous solutions of salts with non-coordinating anions contain the pale-pink, [Mn(H20)6] +, ion which is one of a variety of high-spin octahedral complexes which have been  [c.1060]

It has recently been shown that in some cases, the HTL and ETL materials do not need to be segregated into individual layers to achieve high device efficiency. Eor example, a homogeneous blend can be prepared with a polymeric hole transport (HT) material, and either a molecular or polymeric light emitter and electron transport (ET) material (53,56—61). An OLED prepared in this manner consists of a single organic film sandwiched between an ITO anode and a metal cathode. As in conventional devices, holes are injected from ITO and electrons from the metal cathode, with recombination occurring within the thin polymer film. Polymers and monomeric metal complexes have been used successfliUy in these blended devices to give colors that span the visible spectmm, with external quantum efficiencies >1% (56,57).  [c.244]

Stabihties of inclusion compounds span a wide range. Some are very stable at ambient conditions and requite heating to considerable temperatures or treatment under high vacuum to cause decomposition. Others are only stable when in contact with mother Hquor or excess guest solvent from which the inclusion compound was grown. A simple yet informative way for estimation of inclusion stabihties is to relate the decomposition point of the inclusion compound to the usual boiling point of the respective guest Hquid (110). However, thermodynamic and kinetic properties including stabihties of inclusion compounds have also been studied on a more scientific base (44,128,129). Stability data of crown cation and uncharged molecule complexes of macrocychc and oligocyclic hosts in solution are available in numerous cases (38,39).  [c.74]

Dispersion of pigments for latex paints is the principal appHcation of aqueous dispersions in coatings. Commonly, three surfactants are used in preparing the dispersion of the white and inert pigments potassium tripolyphosphate, an anionic surfactant, and a nonionic surfactant. The final colored latex paint formulations, which are very complex, commonly contain seven pigments, some having high surface energies (inorganic pigments) and some having low surface energies (organic pigments). The latex polymer is present as a dispersion that must be stabilized against coalescence and flocculation, and latexes themselves commonly contain two or more surfactants and a water-soluble polymer. Generally, but not always, the latex particles have a low surface tension. Complexity is increased by the use of at least one and sometimes two water-soluble polymers in the latex paint formula that can adsorb on the surface of some of the pigment particles and, in some cases, the latex particles. Although wetting inorganic pigments with water usually presents no problem, many organic pigments require a wetting agent to displace the air from the surface of the pigment particles. AH dispersions must be stabilized against flocculation (85). The appropriate surfactants, wetting agents, and water-soluble polymers are selected largely by trial and error. Accumulation of data banks of successful and unsuccessful combinations provides a valuable tool for more efficient formulation.  [c.344]

In a multiple-effect evaporator, the effects are numbered in the direction of steam flow, the first effect being the one heated by prime steam. Liquor feed sequence through the evaporator maybe forward, backward, parallel, or mixed. Backward feed (to the coolest effect first and then successively through the higher temperature effects) is generally used when the feed is cold, because only a small volume has to be heated to the highest temperature, thereby reducing sensible heating losses and improving steam economy. Forward feed is generally used when feed is hot and when the concentrated product is not too viscous at the last effect temperature. Where necessary, forward feed can be used on a cold feed and can give almost the same steam economy as backward feed if the feed is preheated in stages by vapor extracted from each higher temperature effect of the evaporator in turn. Such an arrangement is used for seawater, which can be concentrated only in small amounts at high temperature without scaling but about threefold at the lowest effect temperature. These preheaters add to the complexity of the evaporator but not necessarily to total heating surface needs because they reduce the heat loads in the effects themselves. Parallel feed is frequendy used in crystallising operations and involves feeding to and withdrawing product from each effect. Mixed feed operation is common if feed is at some temperature intermediate between first and last effect, the finished product is too viscous to handle at low temperature, or Hquor at an intermediate concentration and temperature is desired for further processing. AH such conditions prevail in a kraft-mill hquor evaporator (see Pulp). The evaporators are usually of the LTV type and the feed heaters are an integral part of the evaporator tube bundles. The highest temperature effect is frequently subdivided so that only a part of the tubes must work on Hquor of the highest concentration and viscosity. Other flow-sheet variations include evaporators with several bodies in parallel on steam and vapor but in series on feed. Thus the kraft-mill evaporator might be an eight-body seven effect with two bodies in parallel in the last effect position in order to better handle the very large vapor volumes generated at the lowest temperature. Variations may also involve the steam path eg, a combination double-triple effect, which may have one effect on prime steam with the evolved vapor being spHt, one part to a single effect and the other to a double effect. This variant is used when the Hquor has a high BPR as it approaches final concentration, so high that not all evaporation can be accompHshed, in this example, in a triple effect. Other flow sheets have been developed for specific types of evaporator, such as parallel spHt feed, which is used for desalination evaporators arranged in vertical stacks. In this case, about half the feed goes to the odd-numbered effects in one stack and the other half to the even-numbered effects in a second stack next to the first, with vapor connections crossing from one stack to the other.  [c.476]

The resistance of Mn to both oxidation and reduction is generally attributed to the effect of the symmetrical d configuration, and there is no doubt that the steady increase in resistance of ions to oxidation found with increasing atomic number across the first transition series suffers a discontinuity at Mn , which is more resistant to oxidation than either Cr to the left or Fe to the right. However, the high-spin configuration of the Mn ion provides no CFSE (p. 1131) and the stability constants of its high-spin complexes are consequently lower than those of corresponding complexes of neighbouring M ions and are kinetically labile. In addition, a zero CFSE confers no advantage on any particular stereochemistry which must be one of the reasons for the occurrence of a wider range of stereochemistries for Mn than is normally found for M ions.  [c.1060]

Table 26.6 CFSE values for high-spin complexes of 10 ions Table 26.6 CFSE values for high-spin complexes of 10 ions
Tris(2,4-pentanedionato)iron(III) [14024-18-1], Fe(C H202)3 or Fe(acac)3, forms mby red rhombic crystals that melt at 184°C. This high spin complex is obtained by reaction of iron(III) hydroxide and excess ligand. It is only slightly soluble in water, but is soluble in alcohol, acetone, chloroform, or benzene. The stmcture has a near-octahedral arrangement of the six oxygen atoms. Related complexes can be formed with other P-diketones by either direct synthesis or exchange of the diketone into Fe(acac)3. The complex is used as a catalyst in oxidation and polymerization reactions.  [c.438]

Iron (III) ethylenediaminetetraacetic acid [15275-07-7] Fe(EDTA) or A/,Ar-l,2-ethanediylbis[A/-(carboxymethyl)glycinato]ferrate(l—), is a pale yellow, high spin complex in which EDTA serves as a hexadentate ligand. A coordinated water molecule is also present, making the iron atom seven-coordinate with a pentagonal bipyramidal stmcture. The stabiUty constant for the formation of the complex is 10. At low pH, a six-coordinate complex is obtained as a result of protonation and decoordination of one carboxylate of the EDTA" ligand. At high pH, )J.-oxo species form and eventually precipitate. The ammonium [21265-50-9] and sodium [15708-41 -5] salts of Fe(EDTA) have been prepared and are used as oxidising agents, especially in photographic bleaching and fixing preparations. The complexes also find use as oxidation catalysts and as a therapeutic source of iron.  [c.439]

Four-coordinate iton(II) porphyrin complexes have the iron atom centered in the plane of the porphyrin, are 5 = 1 intermediate-spin compounds, and can coordinate one or two axial ligands. Five-coordinate iton(II) complexes are square pyramidal where the iron is displaced (up to 50 pm) substantially from the plane. These ate high spin, S = 2 compounds unless the axial ligand is a strong 7T-acceptor, in which case the compound has an 5 = 0 spin state. The iron atom moves back toward or into the porphyrin plane in six-coordinate complexes. These remain high spin with weak field axial ligands, but are low spin for stronger field ligands like amines, pytidines, imidazoles, and cyanide. The two stepwise formation constants for coordination of weak field ligands decrease, permitting five-coordinate complexes to be characterized in solution or isolated. Owing to the spin-state change on coordination of a second strong field ligand, the second formation constants for these ligands ate usually substantially larger than the first. Thus five-coordinate complexes of strong field ligands are not isolated or observed in solution unless the steric bulk of the ligand precludes formation of a six-coordinate complex.  [c.441]

Iron(II) porphyrins react rapidly with O2 to afford p.-oxo-btidged complexes [Fe(III)Por]20 where For is porphyrin. Antiferromagnetic coupling of the two high spin iron atoms reduces the room temperature magnetic moment to about 1.7 x 10 J/T (1.8 /tg /Fe). The reaction involves coordination of O2 to the heme followed by reaction of another equivalent of heme to afford a p.-peroxy-bridged [Fe(III)Por]202 intermediate, homolysis to afford two equivalents of an 0x0 iron(IV) intermediate, and reaction with yet another equivalent of heme to yield the p.-oxo product. Reversible binding of dioxygen, O2, to the heme can occur if steric encumberance of the O2 binding site prevents the approach of a second heme. This is the basis of the success of synthetic O2 binding complexes like picket fence porphyrins. Clearly, one function of the protein surrounding the heme site in the biological oxygen carriers myoglobin and hemoglobin is to isolate the oxygen and prevent its irreversible oxidation. A second function in hemoglobin is to mediate cooperative binding of O2 by the four heme sites in each molecule. The movement of the iron atom with respect to the porphyrin plane upon O2 binding is thought to play an important role in cooperativity.  [c.441]

AH iton(III) porphyrin complexes ate five- or six-coordinate. In five-coordinate complexes the fifth Ligand can be one of a variety of anions including haHdes, carboxyLates, alkyls, alkoxides, phenoxides, pseudohaHdes, and mercaptides among others. Most can be prepared from the conjugate acid of the Ligand and the p.-oxo complex or by metathesis with the chloride complex. TypicaHy, five-coordinate complexes are high spin S = 5/2) and have iron displaced roughly 50 pm from the porphyrin plane toward the Ligand. The dispLacement of iron in compLexes of weak anionic Ligands like CLO decreases to about 25 pm. As a consequence, the d 2 2 orbital is destabilized and the complex adopts an intermediate-spin state. OrganometaHic  [c.441]

Low molecular weight complexes that are synthetic analogues of the protein sites have been prepared and extensively investigated in the cases of stmctures (9), (10), and (12). The compounds, which are typically isolated as tetraalkylammonium salts, assemble spontaneously from a reaction system that includes an iron salt (usually FeCl ), thiolate, a source of labile (elemental sulfur or HS ), and a counterion. Individual compounds can be prepared selectively by variation of the ratios of reactants. Both the iron(II) and iron(III) states of the mononuclear cluster are high spin. Strong intramolecular antiferromagnetic coupling occurs in the cluster compounds, which contain bridging sulfides. Several specific oxidation states of the clusters are mixed valence compounds. [Fe2S2(SR)4] has a localized iron(II) and a localized iron(III). In contrast, localized iron(II) andiron(III) sites are not observed for the [4Fe—4S] compounds, all of which have mixed valent oxidation states. The redox activity of these compounds parallels the states but not the potentials observed in the proteins. Substitution reactions of the terminal thiolate Ligands are also noteworthy.  [c.442]

The examples of piezochromism discussed so far involve rather weU-estabUshed changes of molecular geometry. There are examples of pressure-induced changes in the electronic ground state with resultant changes in the electronic absorption spectmm where the changes in molecular geometry are not weU estabUshed. One example involves intramolecular or intracomplex charge transfer (15—20). At ambient pressure this process takes place by optical excitation. At sufficiently high pressure the charge-transferred state may be stabUized sufficiently to become the ground state of the system with a different electronic absorption spectmm but where the geometry changes are not weU defined. In most transition-metal complexes the metallic ion exists in the state of maximum multiplicity "high spin" according to Hund s rule. With compression the splitting of the d states may become sufficient to estabhsh a spin paired "low spin" ground state with resultant changes in the electronic absorption spectmm (21—23). Again, the changes in molecular geometry are not well estabUshed.  [c.168]

Until now complex compounds of iron and hydroxylamine haven t been received. It is connected with liability of complex compounds of iron with monodentant amines and complication of interaction in the system of Fe-VFe - NH OH. Usual methods that are used to synthesis hydroxylamine complexes in this case turned out to be non-effective. For depression of oxidation-reduction processes and hydrolisation we used the method of synthesis in anaerobic conditions - in atmosphere of argon. Outlet FeCl, was received using the method of high-temperature synthesis. NH OH HCl passed vacuum drying process. Absolute low alcohols were used as solutions. We have received two kinetic unstable complex of iron (II) and hydroxylamine in solution dark-green (pH 3,0 -6,0) (I) and brown-red (pH 6,5 - 7,5) (II). Absorption spectrums of complexes were made, spectral lines were assigned. For the (I) parameter 10 Dg - 9756 cm was found. The character of spectrums shows that I high spin complex of Fe (II) and II is low-spin complex. Using the method A.K. Babko -constants of compounds was counted.  [c.42]

Spin-pairing in manganese(II) requires a good deal of energy and is achieved only by ligands such as CN and CNR which are high in the spectrochemical series. The low-spin complexes. [MnfCN) ] and [Mn(CNR)6] + are presumed  [c.1060]

Further similarity with Mn may be seen in the fact that the vast majority of the compounds of Fe are high-spin. Only ligands such as bipy and phen (already mentioned) and CN , which are high in the spectrochemical series, can induce spin-pairing. The low-spin [Fe(CN)g] , which is best known as its red, crystalline potassium salt, is usually prepared by oxidation of [Fe(CN)6]" with, for instance, CI2. It should be noted that in [Fe(CN)6] the CN ligands are sufficiently labile to render it poisonous, in apparent contrast to [Fe(CN)6]" , which is kinetically more inert. Dilute acids produce [Fe(CN)5(H20)] , and other pentacyano complexes are known.  [c.1090]

Fe " complexes in general have magnetic moments at room temperature which are close to 5.92 BM if they are high-spin and somewhat in excess of 2BM (due to orbital contribution) if they are low-spin. A number of complexes, however, were prepared in 1931 by L. Cambi and found to have moments intermediate between these extremes. They are the iron(lll)-A,A-dialkyldithiocarbamates, [Fe(S2CNR2)3], in which the ligands are  [c.1090]

Iron(II) forms complexes with a variety of ligands. As is to be expected, in view of its smaller cationic charge, these are usually less stable than those of Fe but the antipathy to N-donor ligands is less marked. Thus [Fe(NH3)g] + is known whereas the Fe analogue is not also there are fewer Fe complexes with O-donor ligands such as acac and oxalate, and they are less stable than those of Fe . High-spin octahedral complexes of Fe have a free-ion ground term, split by the crystal field into a ground and an excited  [c.1092]

Mono- and bis-phenanthroline complexes can be prepared but these are both high-spin and, because of the increase in CFSE (p. 1131) accompanying spin pairing ( Aq —> yAq), addition of phenanthroline to aqueous Fe leads almost entirely to the formation of the tris complex rather than mono or bis, even though the Fe is initially in great excess. Pale-yellow K4[Fe(CN)g].3H20 can be crystallized from aqueous solutions of iron(II) sulfate and an excess of KCN this is clearly more convenient than the traditional method of fusing nitrogeneous animal residues (hides, horn, etc.) with iron and K2CO3. The hexacyanoferrate(II) anion (ferrocyanide) is kinetically inert and is said to be non-toxic, but HCN is liberated by the addition of dilute acids.  [c.1092]

Complexes of cobalt(II) are less numerous than those of cobalt(III) but, lacking any configuration comparable in stability with the ijg of Co they show a greater diversity of types and are more labile. The redox properties have already been referred to and the possibility of oxidation must always be considered when preparing Co complexes. However, providing solutions are not alkaline and the ligands not too high in the spectrochemical series, a large number of complexes can be isolated without special precautions. The most common type is high-spin octahedral, though spin-pairing can be achieved by ligands such as CN (p. 1133) which also favour the higher oxidation state. Appropriate choice of ligands can however lead to high-spin-low-spin equilibria as in [Co(terpy)2]X2.nH20 and some 5- and 6-coordinated complexes of Schiff bases and pyridines.  [c.1130]

Magnetic circular dicliroism (MCD) is independent of, and thus complementary to, the natural CD associated with chirality of nuclear stmcture or solvation. Closely related to the Zeeman effect, MCD is most often associated with orbital and spin degeneracies in cliromophores. Chemical applications are thus typically found in systems where a chromophore of high symmetry is present metal complexes, poriihyrins and other aromatics, and haem proteins are  [c.2966]

Cyanides. As a monodentate ligand, the cyanide ion coordinates to metal ions almost exclusively through the carbon atom. In this mode, it is very high in the spectrochemical series, thus most cyanide complexes are low spin (4). As a bidentate ligand the cyanide ion can bridge two metal ions by coordinating to metal ions through either the carbon or nitrogen atoms. Bonding to iron by cyanide involves synergistic CJ-donation and 7T-acceptance by the ligand. Owing to the negative charge, the cyanide anion is a somewhat stronger donor and weaker acceptor than isoelectronic, neutral carbon monoxide (qv) (see CvANmEs).  [c.434]

Iron(II) phthalocyanine [132-16-1] (3), a green compound, was first prepared by accident during the manufacture of phthalimide. Phthalocyanines are an important group of blue/green pigments that have excellent color intensity, photochemical and thermal stabiUty, and chemical inertness (see Phthalocyanine compounds). They find use in dyes, inks, paints, toners, and optical recording media. Iron(II) phthalocyanine is prepared by reductive cyclization of phthalonittile with finely divided iron in a high boiling solvent such as 1-chloronaphthalene and is purified by sublimation at 450°C under partial vacuum. The iron in the complex has a square planar coordination geometry and an intermediate spin, 5 = 1, ground state. The complex is insoluble in most noncoordinating organic solvents, but dissolves in very strong acids such as sulfuric and chlorosulfonic acids owing to protonation of the basic bridging a2a groups. The compound does not dissolve in hot hydrochloric acid, but instead reacts with it to form a material called chloroferric phthalocyanine [14285-56-4] the nature of which is not fully resolved. Iron(II) phthalocyanine forms adducts in coordinating solvents or in the presence of bases, for example phthalocyariinatobis(pyridine)iron [20219-84-5] which can be low spin. Water-soluble iron phthalocyanine complexes are obtained by sulfonating the phenyl residues to obtain tetrasodiumphthalocyaninetetrasulfonatoferrate [41867-66-7]. Purer materials may be obtained, however, by cyclization of sulfonated phthaUc acid or nitrile monomers. Iron(II) phthalocyanine may be reduced by up to four electrons. The complex finds use as a catalyst for a variety of chemical and electrochemical redox reactions.  [c.439]

The magnetic moment arising from (he ground A term is expected to be close to the spin-only value of 3.87 BM and independent of temperature. In practice, providing the compounds are mononuclear, these expectations ate realized rcmarlrably well apart from (he fact that, as was noted for octahedral complexes of vanadium(lll), the third high-energy band in the spectrum is usually wholly or partially obscured by more intense charge-transfer absorption.  [c.1029]

Ruthcniumdll) and osniium([[[) complexes are all octahedral and low-spin wilh I unpaired electron. Iron(lll) complexes, on iho other hand, may be high or low spin, and even though an octahedral stereochemistry is ihe most common, a number of ocher geometries are also found. In other respects, however there is a gradation down the triad, with Ru occupying an mcennediate position between Fe and Os. For iron the oxidation state -l-3 is one of its two most common and for it there is an extensive, simple, cationic chemistry (though the aquo  [c.1088]

The colours of these solutions are of interest. Iron(III) like manganese(II), has a d configuration and its absorption spectrum might therefore be expected to consist similarly of weak spin-forbidden bands. However, a crucial difference between the ions is that Fe carries an additional positive charge, and its correspondingly greater ability to polarize coordinated ligands produces intense, charge-transfer absorptions at much lower energies than those of Mn compounds. As a result, only the hexaaquo ion has the pale colouring associated with spin-forbidden bands in the visible region of the spectrum, while the various hydrolysed species have charge transfer bands, the edges of which tail from the ultraviolet into the visible region producing the yellow colour and obscuring weak d-d bands. Even the hexaquo ion s spectrum is dominated in the near ultraviolet by charge transfer, and a full analysis of the d-d spectrum of this and of other Fe complexes is consequently not possible.  [c.1089]

The most common oxidation states of cobalt are +2 and +3. [Co(H20)6] + and [Co(H20)6] + are both known but the latter is a strong oxidizing agent and in aqueous solution, unless it is acidic, it decomposes rapidly as the Co oxidizes the water with evolution of oxygen. Consequently, in contrast to Co°, Co provides few simple salts, and those which do occur are unstable. However, Co is unsurpassed in the number of coordination complexes which it forms, especially with A-donor ligands. Virtually all of these complexes are low-spin, the configuration producing a particularly high CFSE (p. 1131).  [c.1116]


See pages that mention the term High-spin complexes : [c.275]    [c.204]    [c.273]    [c.7]    [c.469]    [c.1031]    [c.1057]    [c.1095]    [c.1160]    [c.439]    [c.442]    [c.459]    [c.674]    [c.1122]   
Chemistry of the elements (1998) -- [ c.923 ]