Inorganicu Chimicu Acta, 186 (1991) S-10 5 I I Activation volumes for the base hydrolysis of some chloro(diamine)- (triamine)chromium(III) complexes Donald A. House* Department of Chernhtq University of Canterbuy, Christchurch (New Zealand) K. Bal Reddy** and Rudi van Eldik* Institute for Inorganic Chemisq, University of Wittenl Herdecke, 58IO LEtten (F.R. G.) (Received April 22, 1991) The mechanism of base hydrolysis of penta- aminehalochromium(II1) complexes has proved to be difficult to establish in terms of the generally accepted SNiCB mechanism operating for analogous cO(II1) complexes [l-4]. Although the reaction is accelerated by base, rate constants for Cr(II1) are generally much smaller than those for Co(II1) and this has been interpreted in terms of either less acidic protons or a less reactive conjugate base for Cr(II1) [5-71. The recently mea- sured rates of base hydrolysis at ambient pressure of an extensive series of CrCl(diamine)(triamine)‘+ complexes have been interpreted in terms of the conventional SNtCB mechanism [8]. We now report the measurement of the activation volumes (Av) for the base hydrolysis reaction as well as the partial molar volumes (@ for the cations used. Interpretation of these data, when taken in conjugation with AVr and P for CrCl(NH&‘+ and CrCl(NH2CH3)52+ [9], requires some modification of the previous mechanistic proposal [8]. These mod- ifications give a Cr(II1) to Co(II1) mechanistic com- parison that rests more comfortably than the previous direct equivalence. Experimental+ Materials Most of the compounds used were prepared by established procedures [lo]. When zinc(II)-free solutions were required, the ZnCl,*- salts were converted to N03-.C109- or (C104-)z salts by metathesis. Caution: perchlorate salts are potentially explosive. Chloro(diamine)(I,4,&triazaoctane)chromium(III) tetrachlorozincate(II) salts, / CrCl(AA) (2,3-tri) j- ZnCi,, AA = en, tn Trichloro(l,4,8-triazaoctane)chromium(III), [CrCl,(2,ftri)J was prepared from CrC13.6HZ0 (de- hydrated in boiling DMF) and the free base (Aldrich) [ll]. This material could not be purified by recrys- tallisation and was used directly to prepare [CrCl(AA)(2,3-tri)]ZnC1, (AA= en, tn), following the procedure [lo] described to prepare [CrCl(AA)(dpt)]ZnCl+Anaf. Calc. for [CrCl(tn)(2,3- tri)]ZnC14.HzO: C, 19.07; H, 5.40; N, 13.90. Found: C, 19.04; H, 5.37; N, 13.87%. [CrCl(ibn)(dpt)]ZnC14 was similarly prepared from CrCl,(dpt) and the diamine. Anal. Calc. for [CrCl(ibn)(dpt)]ZnCb: C, 23.37; H, 5.69; N, 13.62. Found: C, 23.40; H, 5.70; N. 13.64%. Chloro(diamine)(S-methyl-lJ,Ptriazanonane)- chromium(III) tetrachlorozincate(II), [CrCl(AA)(Medpt)jZnCI,, AA =en, tn Hydrated chromic chloride, CrC1,.6H20 (17 g) was heated (fumehood) in a stirred solution of DMSO (80 ml) until the temperature of the evolved vapour was 190 “C. The deep purple solution (c. 60 ml) was allowed to cool spontaneously to c. 170 “C and Medpt (10 ml) was slowly added. A small temperature rise occurred and the colour changed to dark green. Spontaneous cooling was continued and at c. 110 “C the diamine (5 ml) was added. A temperature of 110-120 “C was maintained for about 10 min and the colour changed to red-purple. After cooling to about 40 “C, the entire mixture was poured into a solution of ZnClz (20 g) in 6 M HCl (300 ml) and left overnight at room temperature. The crude purple-red product that deposited (5-7 g) was recrystallised from 0.1 M HCl at 60 “C (25 ml/g) by addition of ZnClz (2 g/g) and an equal volume of 12 M HCl followed by ice cooling. Several *Authors to whom correspondence should be addressed. **On leave from the Department of Chemistry, Osmania University, Hyderabad 500004, India. ‘Abbreviations used: en = NHACH,)PHB ibn = NH,C(CH,),CH&H,, tn = NH,(CH@H,, pn = NH&X- (CH,)CH2NH,, Me,tn = NH,CHZC(Me),CH,NH2, NMetn = MeNH(CHJ,NH,, dien =NH,(CH,),NH(CH,),NH2, dpt = NHz(CH,)JWCHJJ’JH,> Medpt = NH2(CH&N- (Me)(CH,),NH,, 2,3-tri = NH,(CH2)2NH(CHZ)JW2. 0020-1693/91/$3.50 0 1991 - Eisevier Sequoia, Lausanne