長尾 宏隆

A facile redox-induced nitrito-to-nitro isomerization occurs in cis-[Ru(NO)(ONO)(bpy)(2)](2+) ({RuNO}(6)-type nitrosyl, bpy=2,2'-bipyridine). At room temperature, the one-electron reduction species (cis-[Ru(NO .)(ONO)(bpy)(2)](+)({RuNO}(7))) changes immediately to cis-[Ru(NO .)(NO2)(bpy)(2)](+) ({RuNO}(7)), which can be converted to cis-[Ru(NO)(NO2)(bpy)(2)](2+) ({RuNO}(6)) by one-electron oxidation. The nitro species is an isomeric twin of the original nitrito species. A mechanistic investigation has established that, during the nitrito-nitro redox-induced rearrangement, an oxygen-atom transfer reaction proceeded between the nitrosyl and the adjacent nitrite Ligands. Such a behavior could not be found in the thermally-induced nitrito-nitro rearrangement of the {RuNO}(6)-type nitrosyl complex mentioned above. The {RuNO}(7)-type nitrosyl complex appears to behave as a key intermediate species of the oxygen-atom transfer reaction.

A series of copper(II) complexes with tripodal polypyridylmethylamine ligands, such as tris(2-pyridylmethyl)amine (tpa), ((6-methyl-2-pyridyl)methyl)bis(2-pyridylmethyl)amine (Me(1)tpa), bis((6-methyl-2-pyridyl)methyl)(2-pyridylmethyl)amine (Me(2)tpa), and tris((6-methyl-2-pyridyl)methyl)amine (Me(3)tpa), have been synthesized and characterized by X-ray crystallography. [Cu(H2O)(tpa)](ClO4)(2) (1) crystallized in the monoclinic system, space group P2(1)/a, with a = 15.029(7) Angstrom, b = 9.268(2) Angstrom, c = 17.948(5) Angstrom, beta = 113.80(3)degrees and Z = 4 (R = 0.061, R(w) = 0.059). [CuCl(Me(1)tpa)]ClO4 (2) crystallized in the triclinic system, space group <P(1)over bar>, with a = 13.617(4) Angstrom, b = 14.532(4) Angstrom, c = 12.357(4) Angstrom, alpha = 106.01(3)degrees, beta = 111.96(2)degrees, gamma = 71.61(2)degrees, and Z = 4 (R = 0.054, R(w) = 0.037). [CuCl(Me(2)tpa)]ClO4 (3) crystallized in the monoclinic system, space group P2(1)/n, with a = 19.650(4) Angstrom, b = 13.528(4) Angstrom, c = 8.55(1) Angstrom, beta = 101.51(5)degrees and Z = 4 (R = 0.071, R(w) = 0.050). [CuCl(Me(3)tpa)][CuCl2(Me(3)tpa)]ClO4 (4) crystallized in the monoclinic system, space group P2(1)/a, with a = 15.698(6) Angstrom, b = 14.687(7) Angstrom, c = 19.475(4) Angstrom, beta = 97.13(2)degrees, and Z = 4 (R = 0.054, R(w) = 0.038). All the Cu atoms of 1-4 have pentacoordinate geometries with three pyridyl and one tertiary amino nitrogen atoms, and a chloride or aqua oxygen atom. Nitrite ion coordinated to the Cu(II) center of Me(1)tpa, Me(2)tpa, and Me(3)tpa complexes with only oxygen atom to form nitrito adducts. The cyclic voltammograms of [Cu(H2O)(Me(n)tpa)](2+) (n = 0, 1, 2, and 3) in the presence of NO2- in H2O (pH 7.0) revealed that the catalytic activity for the reduction of NO2- increases in the order Me(3)tpa << Me(2)tpa << Me(1)tpa < tpa complexes.

The complexes [Ru(bipy)(2)(napy-N)(MeCN)][PF6](2) 1 and [Ru(bipy)(2)(napy-N,N')][PF6](2) 2 (bipy = 2,2'-bipyridine, napy = 1,8-naphthyridine) were prepared, and their crystal structures determined by X-ray analysis. The crystal structure of 1 displays an octahedral co-ordination with monodentate napy, acetonitrile and two chelating; bipy. Despite the inequivalency of two nitrogens of napy in 1 in the solid state, the H-1 NMR spectra in the aromatic region resemble those of 2 over the range -90 to 60 degrees C, which implies dynamic behaviour of napy in 1 in solution. Both 1 and 2 were reduced irreversibly at -0.98 V (vs. Ag-AgCl) in dimethylformamide at -20 degrees C, and the process gradually becomes a reversible redox reaction on increasing the temperature to 30 degrees C. An EPR study revealed that one-electron reduction of 1 takes place in the napy-localized orbital without appreciable increase in electron density on one of the nitrogens of napy. The distinct inequivalence in the charge density between the two nitrogen atoms of singly reduced napy results in stabilization of the N rather than N,N' co-ordination mode.

The norbornadiene(terpyridine) complex [Mo(CO)(C7H8)(terpy)I]I (2a) is prepared in two steps from [Mo(CO)(4)(C7H8)] (1), iodine and 2,2':6',2 ''-terpyridine. On treatment with either KPF6 in methanol or AgSbF6 in CH2Cl2 the more soluble salts [Mo(Co)(C7H8)(terpy)I]PF6 (2b) and [Mo(CO)(C7H8)(terpy)I]SbF6 (2c) are isolated with good yields. With 4,4',4 ''-tri-tert-butyl-2,2':6',2 ''-terpyridine, in similar reactions the tert-butyl-substituted terpyridine compounds [Mo(CO)(C7H8)(4,4',4 ''-(t)Bu(3)terpy)I]X (X = I(3a), PF6(3b) or SbF6 (3c)) can be obtained, while the oxidation of 1 with two equivalents of CuBr2 leads to the bromo(terpyridine) complexes [Mo(CO)(C7H8)(terpy)Br]X (X = Br(4a), PF6 (4b) or SbF6 (4c)). The X-ray structural analysis for 4c reveals an unusual arrangement for the ligands around the metal center. A triangle formed by the norbornadiene and the halogen is perpendicular to the plane formed by terpyridine and CO. The iodo compound 2a as well as the bromo analogue 4a reacts with a double quantity of AgSbF6 by halogen abstraction to give a complex of the composition {[Mo(CO)(C7H8)(terpy)](2)(acetone)}(SbF6)(4) (5). Treatment of 5 with two equivalents of PMe(3) affords the phosphine complex [Mo(CO)(C7H8)(terpy)PMe(3)](SbF6)(2)-acetone (6). The similar compound [Mo(CO)(C7H8)(terpy)NCCH2CH3](SbF6)(2) (7), which can be easily prepared from 5 and a fivefold excess of propionitrile, exists in acetone in a concentration-dependent equilibrium with 5. In the presence of NaN3 or NaCl, 5 yields the complexes [Mo(CO)(C7H8)(terpy)X]SbF6 (X = N-3 (8) or Cl(9)). Whereas the azido compound 8 seems to be stable towards coordinating solvents, the chloro complex 9 slowly reacts in acetone.

An oxygen transfer between the NO and the ONO ligands occurs in cis-[Ru(NO)(ONO)(bpy)(2)](2+) when the complex undergoes a redox-induced nitrito-to-nitro linkage isomerization. Such an oxygen transfer does not occur in a thermally-induced linkage isomerization reported previously.

The title complexes catalyze the reductive disproportionation of CO2 forming CO and CO32- in the electrochemical CO2 reduction in CH3CN with LiBF4. The same reduction in the presence of Me(4)NBF(4) instead of LiBF4 produces CH3C(O)CH3 and CH3C(O)CH2COO- in addition to HCOO- and CO in CH3CN/DMSO (1:1, v/v). In this reaction Me(4)N(+) functions as the methylation agent of the carbonyl moiety resulting from the reductive disproportionation of CO2. The produced CH3C(O)CH3 undergoes the abstraction of its ex-proton and subsequent carboxylation to afford CH3C(O)CH2COO-.

Three new nitro-nitrito isomeric pairs are prepared and characterized as cis-[Ru(NO)X(2,2'-bpy)(2)](2+), Cis-[Ru-(NO)X(ppca)(2)], and cis-[Ru(NO)X(2,2'-bpy)(py)(2)](2+) (X = ONO, NO2; 2,2'-bpy = 2,2'-bipyridine; pyca = pyridine-2-carboxylate; py = pyridine). Molecular structures of the isomers are established by X-ray structure studies, except for cis-[Ru(NO)(ONO) (pyca)(2)]. Redox-induced linkage isomerization occurs in cis-[Ru(NO)X(2,2'-bpy)-py)(2)](2+) (X = ONO, NO2); the nitrito isomer is capable of being interchanged to the nitro isomer, via a one-electron redox process of the (RuNO)(3+) moiety. Thermally-induced isomerization also occurs, with different isomerization patterns, depending on the spectator ligands: cis-[Ru(NO)X(2,2'-bpy)(2)](2+) gave an equilibrium mixture of the nitro and the nitrito isomers; in cis-[Ru(NO)X(pyca)(2)], the nitrito isomer changed to the nitro isomer; contrastively, the nitro isomer of cis-[Ru(NO)X(2,2'-bpy) (py)(2)](2+) converted to the nitrito isomer. Some mechanistic investigations about the isomerization reactions were carried out using various N-15-substituted complexes; no oxygen exchange reaction between the nitrosyl and the nitro (nitrito) ligands was found.

A one-electron oxidation species of the title complex, [Ru(NO)Cl-5](-) ((RuNO)(5)-type nitrosyl complex), converted into cis-[Ru(NO)Cl-4(CH3CN)](-) ({RuNO}(6)-type nitrosyl complex) at low temperature (5 degrees C) in the dark. The product species could be isolated and characterized. An X-ray structure determination showed a remarkably short N-O bond distance of the Ru-NO moiety: l(N-O)=0.998 Angstrom, l(Ru-N)=1.787(5) Angstrom, angle Ru-N-O=175.1(6)degrees. Under room light at 25 degrees C, the complex underwent a facile nitrosyl photoelimination to give trans-[RuCl4(CH3CN)(2)](-), the structure of which was also established. During the course of the reactions, two other unidentified species were confirmed to exist by cyclic voltammetry.

Both [Ru(bpy)(2)(CO)(CHO)](+) and [Ru(bpy)(trpy)(CHO)](+) (bpy = 2,2'-bipyridine; trpy = 2,2':6',2 ''-terpyridine) were characterized. The latter is a key intermediate in four- and six-electron reductions of CO2 producing HCHO, CH3OH, HOOCCHO, and HOOCCH2OH. The reactivity of these formyl complexes toward CO2 reveals a new pathway of HCOOH formation in electrochemical CO2 reduction.

The reaction of [Mo(CO)(4)(C7H8)] (1) With I-2 gave the norbornadienemolybdenum(II) complex [Mo(CO)(2)(C7H8)I-2](n greater than or equal to 1) (2), which existed in an equilibrium of two isomeric forms. In acetonitrile, 2 reversibly formed the adduct [Mo(CO)(2)(C7H8)(NCCH3)I-2] (3), whereas on treatment with 2,2'-bipyridine or 4,4'-di-2,2'-tBu-bipyridine, it gave stable 7-coordinated molybdenum(II) complexes, [Mo(CO)(C7H8)(C10H8N2)I-2] (4) and [Mo(CO)(C7H8)(C(10)H(6)tBu(2)N(2))I-2] (5), in good yield. In similar reactions, the related dibromomolybdenun compounds [Mo(CO)(C7H8)(C10H8N2)Br-2] (6) and [Mo(CO)(C7H8)(C(10)H(6)tBu(2)N(2))Br-2] (7) were prepared by oxidation of 1 with two equivalents of CuBr2. The X-ray structural analysis of 6 reveals that the geometry around the molybdenum atom is nearly perfectly pentagonal bipyramidal, with the CO and one of the bipyridyl rings perpendicular to the plane formed by the other ligands. The compounds 5 and 6 react with AgSbF6 by halogen abstraction to give cationic complexes, {[Mo(CO)(C7H8)(C10H8N2)Br]SbF6}(n greater than or equal to 1) (8) and {[Mo(CO)(C7H8)(C(10)H(6)tBu(2)N(2))I]SbF6}(n greater than or equal to 1) (9): In acetone, 8 and 9 reversibly formed the adducts [Mo(CO)(C7H8)(C10H8 N-2)(acetone)Br]SbF6 (8') and [Mo(CO)(C7H8)(C(10)H(6)tBu(2)N(2))(acetone)I]SbF6 (9'); while on treatment with PMe(3), the stable monomeric complexes, [Mo(CO)(C7H8)(C10H8N2)(PMe(3))Br]SbF6 (10) and [Mo(CO)(C7H8)(C(10)H(6)tBu(2)N(2))(PMe(3))I]SbF6 (11), were isolated in almost quantitative yield. In the presence of KBr, compound 8' reverted to the dibromo complex 6, whereas 9' reacted to produce a 1:1:2 mixture of 5, 7 and the bromo(iodo) complex [Mo(CO)(C7H8)(C(10)H6tBu(2)N(2))BrI] (12). The same mixture is available from the reaction of 5 with one equivalent of 7.

Nitrosyl complexes which have both 2,2'-bipyridine and pyridine as co-existing ligands were synthesized and characterized as cis-[Ru(NO)(X)(bpy)(py)(2)](z+) (X = OH, Cl, NO2 for z = 2; X = py for z = 3). Their characteristics were investigated under the conditions of both chemical oxidation and electrochemical reduction, The molecular structure of cis-[Ru(NO) (OH) (bpy) (py)(2)] (PF6)(2) was determined: RuC20N5H19O2P2F12, FW = 752.40, orthorhombic, a = 15.942(2), b = 26.541(4), c = 12.670(5) Angstrom, V = 5360(1) Angstrom(3), space group Pbca, Z = 8, D-calc = 1.864 g cm(-3), D-obs = 1.857 g cm(-3), mu(Mo K alpha) = 8.18 cm-(1), no. of observations (I > 3.00 sigma(I)) = 2232, R = 0.050, R(w) = 0.042. The related nitro and oxo complexes which were obtained from the nitrosyl complexes are also reported.

Triangular metal-sulfide clusters, [{Ir(C(5)Me(5))}(3)(mu(3)-S)(2)](2+) and [{Co(C(5)H(4)Me)}(3)(mu(3)-S)(2)](2+), catalyse the electrochemical CO2 reduction to selectively produce oxalate at -1.30 and -0.70 V (vs. Ag/AgCl), respectively, in MeCN.

The title complex catalyzes reductive disproportionation of CO2 to afford CO and CO32- in the electrochemical CO2 reduction in the presence of LiBF4, while the same reduction in the presence of (CH3)(4)NBF4 in DMSO/CH3CN produced CH3COCH3, CH3COCH2COO-, and HCOO- as well as CO and CO32-.

Copper-nitrito and -nitro isomers, [Cu(ONO)(tpa)]PF6 and [Cu(NO2)(tpa)]PF6 (tpa = tris[(2-pyridyl)-methyl]amine) were isolated and the molecular structures were determined by X-ray analysis. [Cu-(ONO)(tpa)]PF6 (C18H18N5O2PF(6)CU) crystallizes in the monoclinic space group P2(1)/a with a = 13.374(2), b = 14.033(2), c = 13.455(2) Angstrom, beta = 119.10(1)degrees, V = 2206.5(6) Angstrom(3), and Z = 4. [Cu(NO2)(tpa)]PF6 (C18H18N5O2PF6Cu) crystallizes in the orthorhombic space group I/ba2 with a = 16.718(3), b = 17.554(3), c = 14.785(3) Angstrom, V = 4338(1) Angstrom(3), and Z = 8. Those nitrito- and nitro-complexes exist as an equilibrium mixture in solutions. Electrochemical reduction of NO2- in the presence of [Cu(H2O)(tpa)](ClO4)(2) at -0.4 V in H2O (pH 7.0) catalytically produced N2O with concomitant evolution of a small amount of NO via the nitro and nitrito adducts.

Electrochemical reduction of CO2 catalyzed by a triangular rhodium complex [(RhCp*)(3)(mu(3)-S)(2)](2+) selectively produced formate and oxalate in the presence of Bu(4)NBF(4) and LiBF4, respectively, under the controlled potential electrolysis at -1.50 V (vs. SCE) in CO2-saturated CH3CN. A solution IR spectrum evidenced the adduct formation between [(RhCp*)3(mu(3)-S)(2)](0) and CO2 as the possible precursor for the oxalate formation.

A carbonyl ligand of [Ru(bpy)2(CO)2](PF6)2 (1) (bpy = 2,2'-bipyridine) or [Ru(bpy)(trpy)(CO)](PF6)2 (2) (trpy = 2,2':6',2''-terpyridine) is reversibly converted to hydroxycarbonyl and eta1-CO2 moieties by treatment with OH-. 1 and 2 also react with NaBH4 to afford CH3OH via formyl and hydroxymethyl complexes, and the molecular structures of 2 and [Ru(bpy)2(CO)(CH2OH)]PF6 (3) were determined by X-ray structure analysis. Crystal data: 2, C26H19N5OP2F12Ru, monoclinic, space group C2/c, a = 34.683(3) angstrom, b = 10.168(2) angstrom, c = 24.640(3) angstrom, beta = 133.35(1)-degrees, V = 6318(1) angstrom3, Z = 8, and R = 0.046 (R(W) = 0.060) for 5844 data with F. > 3sigma(F(o)); 3, C22H19N4O2-PF6Ru, monoclinic, space group C2/c, a = 30.931(4) angstrom, b = 7.487(1) angstrom, c = 24.873(3) angstrom, beta = 124.68(1)-degrees, V = 4736(1) angstrom3, Z = 8, and R = 0.059 (R(W) = 0.070) for 2880 data with F(o) >3sigma(F(o)). The controlled-potential electrolysis of 2 at -1.75 V vs Ag\Ag+ in CO2-saturated C2H5OH/H2O (8:2 v/v) at -20-degrees-C produced HC(O)H, CH3OH, H(O)CCOOH, and HOCH2COOH together with CO and HCOOH, while the electrochemical CO2 reduction in the presence of 1 gave only CO and HCOOH under similar electrolysis conditions. The achievement of the multielectron reduction of CO2 by 2 as the first example in homogeneous reactions is ascribed to [Ru(bpy)(trpy)(CHO)]+ formed by two-electron reduction of 2 in protic media.

Electrochemical reduction of NO2- by [Cu(tpa)(H2O)](ClO4)2 (tpa = tris[(2-pyridyl)methyl]amine) under the controlled potential electrolysis at -0.40 V (vs. Ag/AgCl) in H2O (pH 7.0) catalytically produced N2O with concomitant NO evolution. As the precursor of NO evolution, [Cu(tpa)(ONO)]PF6 was characterized by X-ray crystallography.

The adamantanethiolate ligated Fe4S4 cluster, [Fe4S4(SAd)4]2-, exhibits two stable [Fe4S4]2+/+ and [Fe4S4]3+/2+ redox couples in dry DMF, while [Fe4S4(SAd)4]- readily undergoes a hydrolysis reaction by the addition of a small amount of H2O in DMF (3 vol.%). The hydrolysis of [Fe4S4(SAd)4]- can be effectively depressed by the presence of free AdSH in H2O/DMF, and also by solubilization in aqueous PDAH solutions (PDAH=poly[2-(dimethylamino)hexanamide]). The observation that the crystal structure of (Ph4As)2[Fe4S4(SAd)4] has enough space for coordination of H2O to the Fe4S4 core indicates that stabilization of [Fe4S4(SAd)4]- in aqueous media is ascribed to depression of dissociation of AdS- from the [Fe4S4]3+ core rather than hydrophobic spheres around the [Fe4S4]3+ core.

The molecular structures of [Ru(bpy)2(CO)2](PF6)2, [Ru(bpy)2(CO)(C(O)OCH3)]B(C6H5)4.CH3CN as a model complex of [Ru(bpy)2(CO)(C(O)OH)]+, and [Ru(bpy)2(CO)(eta1-CO2)].3H2O have been determined by X-ray analysis. The observation that the Ru-C(O)OCH3 bond distance of[Ru(bpy)2(CO)(C(O)OCH3)]+ is shorter than the Ru-CO2 one of (Ru(bpy)2(CO)(CO2)] suggests that the multibond character of the Ru-CO2 bond is not larger than that for Ru-C(O)OCH3. One extra electron pair involved in [Ru(bpy)2(CO)(CO2)] resulting from dissociation of a terminal proton of[Ru(bpy)2(CO)(C(O)OH)]+ may be mainly localized in the CO2 ligand rather than delocalized over the RuCO2 moiety, and the extended three-dimensional network of hydrogen bonding between the CO2 ligand and three hydrated water molecules compensates the increase in the electron density of the CO2 moiety of [Ru(bpy)2(CO)(CO2)].3H2O.

In order to evaluate the acidity of CO2 in protic media, interaction of CO2 with reduced 2,3,5,6-tetramethylquinone (TMQ) was investigated by means of cyclic voltammetry in CH3CN, CH3OH, and CH3CN / H2O. Predominant carboxylation of TMQ in CH3OH and CH3CN / H2O (9:1 v/v) indicates that the acidity of CO2 is almost equivalent or stronger than that of proton in those media.

The molecular structure of the title compound as a model for [Ru(bpy)2(CO)(C(O)OH)]+ was determined in order to elucidate the structural difference between [Ru(bpy)2(CO)(C(O)OH)]+ and [Ru(bpy)2(CO)(eta1-CO2)], both of which are possible reaction intermediates in electrochemical and photochemical CO2 reduction.

Stability of superoxidized form of [Fe4S4(SAd)4]2- (AdS-: 1-adamantanethiolate) in DMF, H2O/DMF, and aqueous poly[2-(dimethylamino)hexanamide] (PDAH) solutions is discussed in connection with the crystal structure of (Ph4As)2[Fe4S4(SAd)4].

A controlled potential electrolysis at -1.55 V versus SCE of CO2-saturated CH3CN containing (Et4N)3[Mo2Fe6S8(SEt)9], CH3C(O)SEt, Bu4NBF4, and Molecular Sieve 3A as a desiccant produced CH3C(O)COO- with a current efficiency of 27%. Similar electrolysis using C2H5C(O)SEt and C6H5C(O)SEt also catalytically afforded C2H5C(O)COO- and C6H5C(O)COO-with current efficiencies of 49 and 13%, respectively. These reactions are strongly inhibited by the presence of not only H2O but also excess EtS-. Strong acylating agents such as acetyl chloride, acetic anhydride, acetyl-sulfide, and acetylimidazole in place of CH3C(O)SEt caused decomposition of [Mo2Fe6S8(SEt)9]3-, and no CH3C(O)COO- was formed under the same electrolysis conditions.

A reaction of [Ru(bpy)2(CO)2](PF6)2 with 2 equiv of Bu4NOH in H2O/EtOH (1:1 v/v) affords an eta(1)-CO2 complex, [Ru(bpy)2(CO)(COO)].3H2O. An addition of an aqueous HCl solution to a MeOH solution of [Ru(bpy)2-(CO)(COO)].3H2O quantitatively regenerates [Ru(bpy)2-(CO)2]2+.

The electrochemical behavior of trans-[Ru(IV)Cl(O)(py)4]+, and that of its related complexes, trans-[Ru(III)Cl(OH)(py)4]+ and trans-[Ru(II)Cl(H2O)(py)4]+, were investigated in both acetonitrile and aqueous solvents. The reduction process of trans-[Ru(IV)Cl(O)(py)4]+ was an irreversible one; it converted into trans-[Ru(II)Cl(OH)(py)4]0 in CH3CN and trans-[Ru(II)Cl(H2O)(py)4]+ in aqueous solvent by a one-step two-electron reduction. The oxo complex undergoes a one-electron oxidation to give a reactive trans-[Ru(V)Cl(O)(py)4]2+, which is the species capable of oxidizing organic substances.

Trans-[Ru(NO2)2(py)4] is oxidized chemically to give trans-[Ru(ONO)(O)(py)4]+, via the formation of a precursor species, trans-[Ru(NO2)(H2O)(py)4]+. The process was confirmed by electrochemical investigations.

The controlled potential electrolysis of CO2-saturated CH3C N containing (Bu4N)3[Mo2Fe6S8(SEt)9], methyl acrylate, Bu4NBF4, and molecular sieves 4A at -1.60- -1.70 V vs. SCE gave -OOCCH2CH(C(O)OCH3)COO-, -OOCCH2CH2CH2C(O)OCH3, CH3CH(C(O)OCH3)COO-, and CH3CH2C(O)OCH3. The formation of those products may be explained in terms of a nucleophilic attack of either activated CO2 or H+ on the two-electron reduced cluster, followed by an electrophilic attack of free CO2 or H+ to olefinic carbons.

Nitro–Nitrito isomerization occurs reversibly when a formal oxidation state of the central metal atom is changed; a nitro complex of Ru(II) gives a nitrito complex of Ru(IV), which is capable of being returned to the original complex by a moderate reduction.