Carbonato Ligand Research Articles

Synthesis, structure and magnetism of a μ 3-carbonato bridged nickel(II) complex with 2,2′,2″-tris(2-aminoethyl)amine ligand: a new coordination mode of carbonato bridge

A novel μ 3 - CO 3 2 - bridged Ni(II) complex with tris(2-aminoethyl)amine (tren), [Ni 3(tren) 3(H 2O) 2(μ 3-CO 3)](ClO 4) 4 · 2H 2O, was obtained by bubbling CO 2 into an aqueous solution of μ 2-OH bridged Ni(II) complex. The X-ray structure analysis of the title complex shows that the three nickel atoms are asymmetrically bridged by one tetradentate carbonato ligand, which presents a new bridge mode. The further magnetochemistry study exhibits a strong antiferromagnetic coupling between nickel(II) ions with J 1 = −11.03 cm −1, J 2 = −80.27 cm −1 in the title compound.

A totally unexpected synthesis of single crystals of the mineral Chalconatronite, Na2[Cu(Co3)2]·3h2o, from a solution of a Copper coordination compound and Atmospheric Co2, at room temperature

In an effort to produce additional polymorphs of [Cu(HL)]2)(ClO4)2 · 2H2O (V) by dissolving that compound in slightly basic, deionized water, we obtained a beautiful set of crystals, obviously containing Cu(II), which turned out to have the composition Na2[Cu(CO3)2]·3H2O. The fact that a chemical reaction occurred while waiting for crystals of the parent compound to form was unknown to us until we solved the crystal structure of the royal-blue crystals. This substance has the interesting property of being a naturally occurring mineral and has also been prepared in the laboratory but by an appropriate mixing of the substance’s components, in basic solution. Each copper contains a bidentate carbonato ligand with one oxygen bridging an adjacent copper. The oxygen that is not bound to the first copper is bonded to two adjacent sodium ions, thus making Na–O–C–O–Cu bridges. Copper occurs in infinite strings formed by carbonato oxygens that bridge the Cu(II) cations. There are also Cu–H2O–Na and Na–H2O–Na bridges in the lattice. The copper and sodium ions are far from ideal octahedral coordination, with large deviations of the bond lengths and angles from those of that geometry. A rational synthesis of the compound was carried out to obtain a significant amount of this compound for physical measurements, such as magnetic susceptibility. VI behaves almost as a simple paramagnet throughout the 35–300 K temperature range and the data points can be fitted to the Curie–Weiss law with θ = 2.05°K. In that temperature range, the compound is slightly antiferromagnetically coupled (typical μ = 1.58 BM at 200 K). However, below ca. 35 K the unpaired electrons of the Cu(II) cation couple ferromagnetically, and at 2 K the magnetic moment is 2.27 BM.

Template synthesis of [(Mo(V)2O4)(O3PCH2PO3)]n clusters (n = 3, 4, 10): solid state and solution studies.

The influence of three exogenous ligands (acetate, formate and carbonate) on the condensation process of the [Mo2O4]2+ dioxocation with the [O3PCH2PO3](4-) group has been investigated. Four cyclic or bicyclic compounds have been isolated and characterized by X-ray diffraction studies. Two closely related acetato and formato ovoidal duodecanuclear compounds, Na24[Na4(H2O)6[(Mo2O4)10(O3PCH2PO3)10(CH3COO)8(H2O)4]].103H2O (1) and Na28[Na2[(Mo2O4)10(O3PCH2PO3)10(HCOO)10]].110H2O (2), respectively, have been obtained. Their structures can be described as two interconnected nonequivalent wheels, delimiting a large cavity. When the condensation is performed in similar conditions but replacing carboxylato groups by carbonato ligands, the ellipsoidal octanuclear Na11[Na(H2O)2[(Mo2O4)4(O3PCH2PO3)4(CO3)2]].70H2O (3) compound is isolated. 31P NMR spectroscopic studies have shown that complexes 1 and 3 are stable in solution at room temperature. Nevertheless, on heating an aqueous solution of 3, the Na8[(Mo2O4)3(O3PCH2PO3)3(MoO4)].18H2O (4) complex, free of carbonato groups, is obtained. 4 is a hexanuclear Mo(V) wheel encapsulating a tetrahedral [Mo(VI)O4](2-) anion. Its rational synthesis using a controlled Mo(V)/Mo(VI) ratio is also presented.

Polynuclear complexes of the pendent-arm ligand 1,4,7-tris(acetophenoneoxime)-1,4,7-triazacyclononane.

The ligand 1,4,7-tris(acetophenoneoxime)-1,4,7-triazacyclononane (H(3)L) has been synthesized and its coordination properties toward Cu(II), Ni(II), Co(II), and Mn(II) in the presence of air have been investigated. Copper(II) yields a mononuclear complex, [Cu(H(2)L)](ClO(4)) (1), cobalt(II) and manganese(II) ions yield mixed-valence Co(III)(2)Co(II) (2a) and Mn(II)(2)Mn(III) (4) complexes, whereas nickel(II) produces a tetranuclear [Ni(4)(HL)(3)](2+) (3) complex. The complexes have been structurally, magnetochemically, and spectroscopically characterized. Complex 3, a planar trigonal-shaped tetranuclear Ni(II) species, exhibits irregular spin-ladder. Variable-temperature (2-290 K) magnetic susceptibility analysis of 3 demonstrates antiferromagnetic exchange interactions (J = -13.4 cm(-1)) between the neighboring Ni(II) ions, which lead to the ground-state S(t) = 2.0 owing to the topology of the spin-carriers in 3. A bulk ferromaganetic interaction (J = +2 cm(-1)) is prevailing between the neighboring high-spin Mn(II) and high-spin Mn(III) ions leading to a ground state of S(t) = 7.0 for 4. The large ground-state spin value of S(t) = 7.0 has been confirmed by magnetization measurements at applied magnetic fields of 1, 4 and 7 T. A bridging monomethyl carbonato ligand formation occurs through an efficient CO(2) uptake from air in methanolic solutions containing a base in the case of complex 4.

Kinetics and mechanism of the acid-catalyzed hydrolysis of [carbonatoethylenediamine(L)2cobalt(III)]+ ions

The kinetics of acid-catalyzed hydrolysis of the [Co(en)(L)2(O2CO)]+ ion (L = imidazole, 1-methylimidazole, 2-methylimidazole) follows the rate law −d[complex]/dt = {k1K[H+]/(1 + K[H+])}[complex] (15–30 or 25–40 °C, [H+] = 0.1–1.0 M and I = 1.0 M (NaClO4)). The reaction course consists of a rapid pre-equilibrium protonation, followed by a rate determining chelate ring opening process and subsequent fast release of the one-end bound carbonato ligand. Kinetic parameters, k1 and K, at 25 °C are 5.5 × 10−2 s−1, 0.44 M−1 (ImH), 5.1 × 10−2 s−1, 0.54 M−1 (1-Meim) and 3.8 × 10−3 s−1, 0.74 M−1 (2-MeimH) respectively, and activation parameters for k1 are ΔH1≠ = 43.7 ± 8.9 kJ mol−1, ΔS1≠ = −123 ± 30 J mol−1 deg−1 (ImH), ΔH1≠ = 43.1 ± 0.3 kJ mol−1, ΔS1≠ = −125 ± 1 J mol−1 deg−1 (1-Meim) and ΔH1≠ = 64.2 ± 4.3 kJ mol−1, ΔS1≠ = −77 ± 14 J mol−1 deg−1 (2-MeimH). The results are compared with those for similar cobalt(III) complexes.

Effect of Nonlabile Quadridentate Ligand on the Acid-catalysed Hydrolysis Velocity of Cobalt(III) Carbonato Chelates

Acid-catalysed hydrolysis kinetics were studied for 10 cobalt(III) complexes bearing a bidentate carbonato ligand at oxonium ion concentration of 0.02-1.0 u M, ionic strength of 2.0 (NaClO 4 ) and ...

Why are Hexol Cluster Cations [Tris(cis-tetraaminedihydroxo)Co]+6 So Easily Resolved into Their Antipodes? Part 2. Preparation, Crystal Structural Studies and the Nine Sources of Chirality of Hexol Clusters

Heretofore, it had been impossible to determine the structure of the original Werner's hexol cation inasmuch as the synthetic procedure given by Werner produces crystals unsuitable for single crystal X-ray (XRD) diffraction studies. Thus, it was greatly satisfying that crystals obtained here, using a different method of preparation, produced useful crystals of three different derivatives. X-ray quality crystals of composition [Co(NH3)4CO3][Co{(HO)2Co(NH3)4}3](NO3)6(OH) · 2 H2O(III) were obtained while attempting to grow crystals of [Co(NH3)5CO3]NO3 from water solutions allowed to evaporate at room temperature (ca. 22°C). (III) crystallizes in space group P\(\overline 1\) (No. 2), with lattice constants: a = 11.767(1), b = 12.856(2), c = 17.040(1) A, α = 75.49(2), β = 72.59(2), γ = 68.53(7)°, V = 2259.84 A3, and d(calc; M.W. = 1148.2, Z = 2) = 1.687 g-cm−3. A total of 4428 data were collected using MoKα (λ = 0.71073 A radiation, over the range of 4° ≤ 2θ ≤ 40°; of these, 2469 [independent and with I ≥ 3σ(I)] were used in the structural analysis. Data were corrected for absorption (μ = 18.97 cm−1) and transmission coefficients ranged from 0.536 to 0.999. The final R(F) and RW(F) residuals were, respectively, 0.18 and 0.18, the high values of which are due to disorder of several of the nitrate counteranions. Nonetheless, the results are of interest since this double salt consists of two fully ordered cations: [Co(NH3)4CO3]+, containing a bidentate carbonato ligand, and Werner's hexol cation {Co[Co(OH)2(NH3)4]3}6+. The presence of the bidentate carbonato cation is important in view of the fact that the starting material was the monodentate pentaamine carbonato cation, a result which provides a guide to the mechanism of the reactions involved in the preparation of compound (III), in particular, and of the hexol cations, in general. Thus, this study is an amplification of our recent description [1] of Werner's hexol salts. Finally, we explain here the origin of the large optical rotatory power of these cations and enumerate the nine sources of chirality present in them. In our previous paper, we listed only seven [1]; however, we have recently come to realize there are two additional ones, described below.

Reactivity of Carbon Dioxide with n-Butyl(phenoxy)-, (alkoxy)-, and (oxo)stannanes: Insight into Dimethyl Carbonate Synthesis

The CO2 insertion into Sn−O bonds of a series of butyl(phenoxy)-, (alkoxy)-, and (oxo)stannanes has been investigated. The tributyl derivatives Bu3SnOR (2a, R = Me; 3a, R = iPr; 4a, R = tBu; 5a, R = SnBu3)1 give quantitatively Bu3Sn(OCO2R), 2b−5b; the analogous tributylphenoxystannane, 1, is less reactive. For the dibutyl series, Bu2Sn(OR)2, steric effects of tBu groups in OR (8a) suppress carbonation under atmospheric pressure. With R = Me (6a) or R = iPr (7a), only one Sn−OR bond reacts, resulting in Bu2Sn(OR)(OCO2R), 6b or 7b. Treating 6a with 2-propanol affords under CO2 the mixed compound Bu2Sn(OMe)(OCO2iPr), selectively. Facile deinsertion of CO2 is a common property of all compounds, occurring more readily in the dibutyl series. The stoichiometric transformation of the carbonato ligand in 2b, 5b, or 6b to dimethyl carbonate (DMC) on reaction with MeI requires nucleophilic assistance by F- to proceed. In the presence of MeOH, 2b and 5bare almost inactive forDMC formation, in contrast with 6b. The best yield is obtained under supercritical CO2−methanol conditions.

Triphenylphosphine-substituted N, N-di- iso-propylcarbamate of ruthenium(II) and its reactions with carbon monoxide

Chloride displacement from RuCl 2(PPh 3) 3 by NH i Pr 2/CO 2 in toluene as medium gave the N, N-di- iso-propylcarbamato derivative Ru(O 2CN i Pr 2) 2(PPh 3) 2, ( 1), whose X-ray crystal structure determination showed the mononuclear compound to contain hexacoordinated ruthenium bonded to bidentate carbamato and to cis-arranged tertiary phosphine groups in a distorted octahedral geometry. Carbonylation at room temperature rapidly converted 1 to the monocarbonyl derivative Ru(O 2CN i Pr 2) 2(PPh 3) 2(CO) ( 2), presumably containing a monodentate carbamato group, the dicarbonyl compound Ru(O 2CN i Pr 2) 2(PPh 3) 2(CO) 2 ( 3) being formed over longer reaction times. X-ray diffraction data showed 3 to contain hexacoordinate ruthenium(II) with monodentate carbamato groups trans to the carbonyl groups and with trans-arranged triphenylphosphine groups. Controlled hydrolysis of 3 yielded the dicarbonyl-carbonato complex Ru(O 3C)(PPh 3) 2(CO) 2·H 2O ( 4), crystallographically established to contain a bidentate carbonato group, trans triphenylphosphine ligands and water hydrogen-bonded to the carbonato ligand.

Synthesis, crystal structure and magnetic behaviour of ( μ3-CO 3)[Ni 3(Medpt) 3(NCSe) 4], a new example of trinuclear nickel(II) complex with pentadentate carbonato bridge and strong antiferromagnetic coupling

The trinuclear complex ( μ 3-CO 3)[Ni 3(Medpt) 3(NCSe) 4] was obtained by reaction of basic solutions of nickel(II), Medpt (bis-(3aminopropyl)methylamine) and potassium selenocyanate with carbonate anion. The crystal structure of ( μ 3-CO 3) [Ni 3(Medpt) 3(NCSe) 4] has been solved. The three nickel atoms are asymmetrically bridged by one pentadentate carbonato ligand, which shows the second example of a novel co-ordination mode of the carbonate ligand. The ( μ 3-CO 3)[Ni 3(Medpt) 3(NCSe) 4] compound shows a very strong antiferromagnetic coupling. Fit as irregular triangular arrangement gave the coupling parameters J 1=−82.1, J 2=−67.4 and J 3=−4.3 cm −1.

Kinetics and mechanism of the acid-catalysed hydrolysis of the carbonatotetraimidazolecobalt(III) ion

The kinetics of the acid-catalysed hydrolysis of the [(imidazole)4Co(CO3)]+ ion was found to follow the rate law -dln[complex]/dt = k 1 K[H+](1 + K[H +]) in the 25–45 °C range, [H+] 0.05–1.0 m range and I = 1.0m. The reaction sequence consists of a rapid protonation equilibrium followed by the one-end dissociation of the coordinated carbonato ligand (rate-determining step) and subsequent fast release of the monodentate carbonato ligand. The rate parameter values, k 1 and ITK, at 25 °C are 6.48 × 10−3s−1 and 0.31m −1, respectively, and activation parameters for k 1 are ΔH 1 ≠ = 86.1 ± 1.2kJ mol−1 and ΔS 1 ≠ = 2.1 ± 6.3 J mol−1K−1. The hydrolysis rate increases with increase in ionic strength. The different ways of dealing with the data fit are presented and discussed. The kinetic results are compared with those for the similar cobalt(III) complexes.

A Novel Pentadentate Coordination Mode for the Carbonato Bridge: Synthesis, Crystal Structure, and Magnetic Behavior of (&mgr;(3)-CO(3))[Ni(3)(Medpt)(3)(NCS)(4)], a New Trinuclear Nickel(II) Carbonato-Bridged Complex with Strong Antiferromagnetic Coupling.

The trinuclear complex (&mgr;(3)-CO(3))[Ni(3)(Medpt)(3)(NCS)(4)] was obtained by reaction of basic solutions of nickel(II), Medpt (bis(3aminopropyl)methylamine) and thiocyanate ligand with atmospheric CO(2) or by simple reaction with carbonate anion. (&mgr;(3)-CO(3))[Ni(3)(Medpt)(3)(NCS)(4)] crystallizes in the triclinic system, space group P&onemacr;, with a = 12.107(5) Å, b = 12.535(7) Å, c = 16.169(9) Å, alpha = 102.69(5) degrees, beta = 92.91(5) degrees, gamma = 118.01(4) degrees, Z = 2, and R = 0.043. The three nickel atoms are asymmetrically bridged by one pentadentate carbonato ligand, which shows a novel coordination mode. The (&mgr;(3)-CO(3))[Ni(3)(Medpt)(3)(NCS)(4)] compound shows a very strong antiferromagnetic coupling. Fit as irregular triangular arrangement gave J(1) = -88.4, J(2) = -57.7, and J(3) = -9.6 cm(-)(1), which is the strongest AF coupling observed to date for Ni(3) compounds. The magnetic behavior of the carbonato bridge is discussed.

Structures and Properties of Vanadium(III) Complexes with 1,2-Diaminoethane-N,N′-diacetate-N,N′-dipropionate and N,N-Bis(carboxymethyl)-β-alanate

Abstract The crystal structures of K[V(eddda)]·2H2O (1) and [V(β-alada)(H2O)2]·H2O (2) were determined by X-ray crystallography, where eddda and β-alada denote 1,2-diaminoethane-N,N′-diacetate-N,N′-dipropionate and N,N-bis(carboxymethyl)-β-alanate, respectively. The crystal of 1 is monoclinic, space group P21/n, a = 10.2753(8), b = 11.3007(8), c = 15.278(2) Å, β = 104.857(9)°, V = 1714.7(3) Å3, Z = 4, and R = 0.052. The crystal of 2 is orthorhombic, space group P212121, a = 7.451(1), b = 11.852(2), c = 13.403(2) Å, V = 1183.6(3) Å3, Z = 4, and R = 0.053. Complexes 1 and 2 adopt a hexacoordinate structure in contrast to the heptacoordinate one of the closely related ethylenediamine-N,N,N′,N′-tetraacetate and nitrilotriacetate complexes. Complexes 1 and 2 yield oxo-bridged dinuclear complexes upon base hydrolysis. The reaction of 2 with CO32− results in the formation of the heptacoordinate species with the bidentate carbonato ligand.

Crystal structure of tetraguanidinium tri(carbonato)oxotitanium(IV) dihydrate, [C(NH 2) 3] 4[TiO(CO 3) 3]·2H 2O

The title complex has been prepared and characterized by IR spectroscopy and X-ray crystallography. The crystals are monoclinic, space group P2 1, with a = 9.2010(9), b = 13.044(3), c = 9.888(4) Å, β = 109.35(2)° and Z = 2. The structure has been refined to R F = 0.038 for 288 variables and 3385 observed Mo Kα reflections. The crystal consists of an assembly of [C(NH 2) 3] + cations, [TiO(CO 3) 3] 4− anions, and water molecules which are interlinked by hydrogen bonds to form a three-dimensional network. The coordination geometry about the titanium atom may be described as a distorted pentagonal bipyramid, the axial positions being occupied by the oxo ligand and an oxygen atom of one of the chelating bidentate carbonato ligands.

The phenomenon of conglomerate crystallization. XVIII. Clavic dissymmetry in coordination compounds. XVI

The x-ray crystal structure of {[Co(NH3)4(CO3)]NO3}2 · H2O has been determined as part of a study of the intra- and interionic interactions present in crystals of several transition-metal-amine complexes chosen to examine the occurrence and causes of conglomerate crystallization. {[Co(NH3)4(CO3)]NO3}2 · H2O crystallizes in the monoclinic space group P21/n with cell constantsa=7.4960(9)A,b=22.673(6),c=10.513(1), andβ=91.41(1)°;V=1786.12 A3, andd(calc;Z=4)=1.915 g cm−3. In all, 5333 data were collected over the range of 4°≤ 2θ≤ 60°; of these, 3395 [independent and with /≥3σ(1)] were used in the structural analysis. Data were corrected for absorption (μ=19.361 cm−1) and the relative transmission coefficients ranged from 0.9987 to 0.8013. The data were of a quality such that both ammonia and water hydrogens were found in difference Fourier maps. The finalR(F) andR w(F) residuals were, respectively, 0.0333 and 0.0332. A trans effect is observed for both cations of {[Co(NH3 (CO3)]NO3}2 · H2O. The equatorial nitrogens, trans to the carbonato oxygens, have shorter Co-N distances than the axial nitrogens, trans to one another. The carbonato ligands are not symmetrically bonded to their respective metal centers. The Co-O distances for cation 1 are 1.913(1) and 1.903(1) A and those for cation 2 are 1.916(1) and 1.896(1) A. The structure reveals the existence of an intricate array of hydrogen bonds, involving both the chelating and nonchelating oxygens of the carbonato ligands as hydrogen bond acceptors of the amine hydrogens. The amine hydrogens are also involved in significant hydrogen-bonding interactions with the nitrate oxygens and water of crystallization, although they are generally weaker than those of the carbonato oxygens.

Investigation of the structure and reactivity of cobalt(III) carbonato complexes by using EHMO calculation

Extended Hückel molecular orbital (EHMO) calculations on modified structures of [CoCO 3(NH 3) 4] + have been performed to investigate the influence of the strain caused by a chelated carbonato ligand on the complex structure and on the decarboxylation of carbonato cobalt(III) complexes with tetraamine ligands ([CoCO 3(N) 4] + type). It was suggested that the CO 3 2− chelation enlarges the NCoN angle (θ) trans to the OCoO angle to some extent and in the most stable structure the angle θ of [CoCO 3(NH 3) 4] + is about 94°, which is slightly larger than the octahedral angle (90°). The existence of d electrons in the d xy orbital seems to be a key point for controlling the NCoN angle in this type of complex. Furthermore, the difference in decarboxylation rate between trans-[CoCO 3(NH 3) 2en] + and trans-[CoCO 3(NH 3) 2tn] +, in which en and tn refer to ethylenediamine and trimethylenediamine, respectively, was examined on the basis of the calculated potential energies of the structure-modified [CoCO 3(NH 3) 4] + as a model compound.

Infrared spectroscopic studies of uranyl(VI) species adsorbed from aqueous [UO2(CO3)3]4? solutions on to a polymer bearing amidoxime groups

Infrared spectra of uranyl(VI) species adsorbed from aqueous [UO2(CO3)3]4– solutions on to a polymer having amidoxime groups were examined in order to obtain information on the adsorption mechanism. The subtraction spectra in a region of 500–1 500 cm–1 exhibit only a distinct band at 886 cm–1 ascribed to the ν3 mode of the OUO moiety and no band assigned to carbonato ligands, suggesting the existence of a carbonato-free uranyl(VI) complex; this is consistent with the adsorption equilibrium reported previously.

A correlation between infrared stretching mode absorptions and structural angular distortions for the carbonato ligand in a wide variety of complexes

The i.r. spectra of several new carbonato complexes of cobalt(III) are reported and it has been found that the bridging carbonato ligand can be distinguished from the bidentate species and possibly from the monodentate species. By comparing available X-ray structural and i.r. spectral data for a large variety of carbonato complexes a strong correlation has been found between the angular distortion (from D 3 h ) within the carbonato ligand and the separation between the two highest energy CO stretching modes.

複錯塩〔Ｃｏ（ＮＨ３）６〕〔Ｌｎ（ＣＯ３）３〕・ｎＨ２Ｏ（Ｌｎ＝Ｌａ，Ｓｍ，Ｇｄ，Ｈｏ）の赤外スペクトルと熱分解

Studies on the infrared spectra and thermal properties of [Co(NH3)6][Ln(CO3)3]⋅n H2O [ 1 ], [Co(NH3)6][Sm(CO3)3]⋅5H2O [2], [Co(HN3)3][Gd(CO3)3]⋅5H2O [3], [Co(NH3)6][Ho(CO3)3(CO3)3]⋅4H2O [4] were undertaken. The infrared spectra of carbonato lig ands of these double complex salts were much more complicated than those of free carbonate ion and coordinating carbonates like those in [Co(NH3)5CO3]Br and [Co(NH3)4CO3]Cl, etc. [2], [3] and [4] show the absorptions attributable to v3 vibrational mode beyond 1500 cm-1, whereas does not. Furthermore, dehydration of [2], [3] and [4] were found to o[c1c]u r in three stages. Values of activation energy in these dehydration processes were estimated using modified Doyle's and Wiedemann's methods.

Pore structures and bulk properties of titania gels prepared from titanous chloride

A series of titanium dioxide gels has been prepared by the oxidation of precipitated hydrous Ti(III) oxide, obtained by the addition of aqueous ammonia or ammonium carbonate to aqueous solutions of titanous chloride. The gels were dried in air at 110°C and their structures and surface properties studied with the aid of DTA, X-ray diffraction, nitrogen adsorption, and spectrophotometry. Analysis of the nitrogen isotherms has revealed that the gels exhibited a wide range of different pore structures, extending from narrow micropores to wide mesopores. The development of the gel porosity appears to have been largely controlled by the mode of removal of water ligands from the hydration sphere of the cations, enlargement of the micropore volume being obtained by the replacement of water by carbonato ligands and pore widening by increase in the pH of precipitation (over the pH range 2.3–7.1). All the gels were X-ray amorphous in their original state: some were converted directly to rutile at 500°C while others underwent the more usual transformation into the intermediate anatase form. It is postulated that the rapid removal of the water ligands can result in the formation of a defect structure which controls the development of the pore structure and also the behavior of the gels on heat treatment.