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The hydrothermal deposition of CrO(OH) from aqueous chromium(III) chloride, nitrate, and perchlorate, and of Cr3(SO4)2(OH)5•H2O and Cr(OH)SO4•2H2O(?) from aqueous chromium(III) sulfate, has been investigated. Aqueous chromium(III) is oxidized to chromium(VI) by 0.4 M perchloric acid above 225 °C, and by molecular oxygen above 250 °C. Aqueous chromium(III) can react at 300 °C with iron or steel, cobalt, and copper to produce FeCr2O4, CoCr2O4, and Cu2Cr2O4, respectively. CrO(OH) reacts with type 316 stainless steel at 440 °C in supercritical water of density 0.7 g cm−3 to yield (Fe,Ni)-Cr2O4, which is the "cubic Cr2O3" of Laubengayer and McCune. The spinels MCr2O4 (M = Mg, Mn, Fe, Co) can be made hydrothermally at ca. 300 °C from Cr(OH)3 and M(OH)2. This information is relevant to corrosion phenomena, and the possible hydrothermal origin of chromite deposits in serpentinized rocks.

For the replacement of X-by H 2 0 in Cr(NH3),X2+, AVOx (the value of the volume of activation AV* at zero pressure) at 25.0" in 0.1 m NH,CIOL is -10.8, -10.2, and -9.4 cm3 mol-' respectively for X -= Cl-, Br-, and I-, while A V, the molar volume of reaction, is -8.4, -7.2 and -6.0 cm3 mol-'. The pressure-dependence of AV" can be expressed in terms of the number x of water-molecules which are electrostricted as the systems go from ground-state to transition-state; for X-= C1-, Br-, and I-, x = 1.9, 2.0, and 1.7. These data, combined with AVv = -5.8 cm3 mol-' for the aquo-exchange of Cr(NH,j,-OH,,+, indicate an associative mechanism for these reactions, as does the contrast with AVO* data for the formally analogous, but dissociatively-activated, reactions of CO(NH,),X(~-")-.Dans le remplacement de X -par H z O dans Cr(NH,),X2+, AVO* (valeur du volume d'activation AV* B la pression zkro) a 25.0' dans NH,CIO, 0.1 n1 est de -10.8, -10.2, et -9.4 cm3 mol-' respectivement pour X -= CI-, Br-, et I-, tandis que AV, volume molaire de la reaction, vaut -8.4, -7.2, et -6.0 cm3 mol-'. Z'influence de la pression sur A V* peut Etre exprimee en fonction du nombre x de molecules d'eau, conditionnk electrostatiquement lorsque le systZme passe de l'etat fondamental a l'itat de transition; pour X-= C1-, Br-, et I-, x = 1.9, 2.0, et 1.7. Ces donnees, combintes a AV* = -5.8 cm3 mol-' pour I'Cchange a i'eau de Cr(NH3),0H,3+, indiquent un mecanisme associatif pour ces reactions, de m&me que les donnees contraires sur AVO* concernant les analogues formels, mais actives par dissociation, des rkactions de CO(NH,),X(~-"'+.[Traduit par le journal]Can 3 Chem.. 51. 821 (1973) We have recently shown (1-5) that the effect of pressure P upon the rates of substitution reactions of transition-metal complexes in aqueoi;s solution can lead to useful mechanistic information if either the reaction is suitably simple or there is an independent criterion with which the derived parameters (the volun~e of activation, AV", and its pressure dependence, if significant) can be compared. Thus, for the ideally simple aquo-exchange reaction of CO(NH,),OH,~+, AV* is + 1.2 cm3 mol-' and is pressure-independent within the experimental uncertainty (6), which suggests a dissociative mechanism. This conclusion is reinforced by the observation that for X n -= SO,*-, Cl-: Br-, NO3-, and FI,O in the reaction there is an excellent linear correlation of slope I .O between A V,'$ ( = A Yq' at zero pressure) and the molar volume of reaction AV> which indicates that the Co-X is completely broken, and

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The aquation of Cr(NH3)SN032+ in acidic solutions yields not only Cr(NH3),OHZ3+ (33%) but also more highly aquated species ~ncluding C~~-C~( N H~) , ( O H~)~~ + and Cr(NH3)3(OH2)33+. In 0.1 M HC104, several successive reactions of comparable rates are involved, but at p H 6 only the two competing initial reactions producing Cr(NH3),0H2+ and cis-Cr(NH3),(OH)N03+ are rate-controlling, s o that the overall reaction rate is first-order in substrate with the apparent parameters kA = 2.4 x s-' (25"), AHA* = 20.4 kcal mol-I, and ASA* = -2.1 cal deg-I mol-'. In alkaline solutions, the pseudofirst-order rate of hydrolysis at a given [OH-] is governed by k.,, = kA + koH [OH-], with /coH = 1.1 x M -I s-I (25"), AHoH* = 25.2 kcal mol-I, and ASOH* = 17.0 cal deg-' mol-'. T h e path characterized by koH leads exclusively to Cr(NH3),0HZ+. In both aquation and base hydrolysis, it is the Cr-ONO, bond that is broken. The production of more highly aquated species in the aquation reactions is attributed to transient chelation of NO3-at the expense of a ligand cis to it. The evidence for this mechanism, and for its operation when oxyanions other than nitrate are coordinated to Cr(III), is reviewed.L'aquation du Cr(NH3),N032+ dans des solutions acides a conduit non seulement au Cr(NH3),0HZ3+ ( 3 3 7 3 mais aussi aux especes "aquo" suptrieures telles C~. S -C~( N H~)~( O H~)~~+ et Cr(NH3)3(OH2)33+. Dans le HCIO, 0.1 M, plusieurs reactions successives de vitesses comparables sont impliqukes alors qu'a pH 6, les deux reactions initiales conduisant au Cr(NH,),0H2+ et au cis-Cr(NH3),(OH)N03+ sont les seules a contrbler la vitesse. Par suite, dans ce dernier cas, la vitesse globale de la reaction obeit a une expression de vitesse du premier ordre par rapport a u substrat dont les parametres sont: kA = 2.4 x S-' (25"), AH,* = 20.4 kcal mol-' et ASA* = -2.1 cal deg-' mol-l. Dans des solutions alcalines, la vitesse d'hydrolyse obCit a une expression de vitessedu pseudo premier ordre pour une certaineconcentra- [Traduit par le journal] Can.

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