Equilibrium, Kinetic and Mechanistic Studies of Formation of Cis- mono (AA)bis (oxaJato) Chromate (III) C,omplex (where AA is · glycine, alanine and histidine)In Monderately Aqueous Acidic Solution

Abstract

Equilrium, kinetic  and mechanistic studies  for  thcoordination of

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some amino acids  "'AA'1

glycine,  alanine, .a:ncl  histidine, to  Cr  (Ill)

center  of trans .[Cr(ox}2(B.2 0hr   {TJ'} cornplein monderarely  acidic

range ofpH=4.8-6-.7 ( p =Q.4M NaN03) are  reported.  The equili rium

c.onsta:nts   at  25°C   .were  found   logKequ.=4.95J ,5.206and5.128for glycine, alap.ine, md histidine, ligation. reactions  respectively..  The substitution reaction is kinetically  involve  rwo parallel  complex reactions, each  inch,tdes there  difierent e_onsecutive steps of different

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species of Ct(III).  The  reactant,  Ti, branches  through  byrolysis  into, doubly  charged anion, conjugated    se of  trans[Cr(ox)22(H0)0Hr2

whlch  inleracts  with  (AAH)   to  from  outer-sphere  complex,   that  is followed  by interchange reaction  of one molecule H20 with one end of    AA-        ron   from                   an         active        intermediate         species   of              trans­ [Cr(Qx)2(AA)OHT3           which               cbanges       to        final           product ;                c.is­ [Cr(ox}1(AA)J?. While  the  remai ning  part  of reactant   interacts also

with zwitter i:on (AAH)  on  other  side  to form  another  outer-sphere

.complex  which  also  undergaes interchange  reaction of one  molecule

H10 wHh AA- ion to ive two· active  intennediates species, trans and cis-[Cr(ox)2(AA)H2 or, whlch later convert tp the same prod·uct .; cis­

fCr(ox)z(AA)]2-.      TI1e      interchange      rate      co.nstant    of     trans­ [Cr(ox)2(AA)OH]                                             formation  relatively   appears  much  larger  than (A H*=8.8kc 1/n1ol)         that                                 of    ds[Cr(.oxh  AA)H:z Ot                     formation

( l\ H*·=14.0Kca l/mol), wbile  the latter rate constant'is oeaily  (5-10)

time l(!I'ger than that oftrans-{Cr(ox)2(AA)H20t fennatlon