Indian Journal of Chemistry
Vol. 38A, July 1999, pp.686-69I
Mechanisms of the acid catalysed bridge cleavage reactions of two trinuclear
ruthenium(lII) complex ions [Ru3(NH3)I/SCN)X2]6+, (X = CI-', Br-)
Biswanath Chakravarty*, Ruma Bhattacharya, Shambhunath Bisai & Md. Munsur Rahman
Department of Chemistry, University of Kalyani, Kalyani 741 235, India
Received 22 Jalll/ary 1999; revised 13 ApriL 1999
Acid catalysed bridge cleavage reaction of two doubly bridged trinucIear complex ions [Ru)(NH) IJ(SCN)X1J"', (X= Cl-, B,) have
been investigated at 59" in perchloric and nitric acid media respectively. Both the complexes show Iwo stages of reaction. Plots of k nt><
versus [acid] are linear for both the complexes in the two stages of reactions. Only the plots for first stage have intercept on rate axis.
The rate constants for the chloro and the bromo complexes for acid - dependent path are as follows : tirst stage 5.15 x 10-' &
2.43 xl 0'" dm) mol- I S-I and second stage 7.35 x 10-) & 4.02 xl Q-4 dm) mol -1s- 1 respectively. An associative mechanism has been
invoked for the reactions in which bond formation by the incoming solvent (water) molecule with the middle ruthenium atom leads to
rupture of bridging bonds.
Sykes and other workers have initiated studies on the
acid and base catalysed bridge cleavage reactions of
dinuclear complexes of cobalt(III) and chromium(III)I .
Studies on the acid and base catalysed bridge cleavage
reactions of ruthenium(III) complexes are Iimited 2-6 .
Studies on the bridged complexes having two different
me tal ions as ruthenium(II) and rhodium(lII) or
iridium(III), have also been reported 7 . In the present investigation, we have reported bridge cleavage reaction s
of two trinuclear complexes of ruthenium(III) in acidic
medium. The complexes have thiocyanate and chloride,
and thiocyanate and bromide bridges res pective ly.
Mechanisms of bridge cleavage in a doubly bridged
trinuclear complex might be interesting as this can indicate the order of cleavage of bridges in a multiply bridged
complex and in thi s regard the present one would be an
interesting addition.
The experimental complexes are:
Materials and Methods
Preparation of complexes
The two complexes under investigation,
[Ru 3(NH')13(SCN)CI 2]CI 6 and [Ru/NH3)13(SCN)BrJBro
were prepared according to literature method x. Their
purity was checked by elemental analysis and spectral
matching with authentic samples.
All the chemicals used in thi s study were of analar
grade. Kinetic runs were followed spectrophotometrically using a SICO uvispec spectrophotometer (mode l
no. 100). Reactions were initiated by adding requi site
amounts of the complex to thermall y pre-equilibrated
solutions of respective acids, HCl0 4 , HN0 3 or PTS and
their sodium salts (for maintaining ionic strength). Sample quenching technique was adopted for absorbance
measurements at suitable time interval s. The kinetics
were followed initially at 488 nm and later at 400 nm
(4 12 nm for Br- complex) . All the kinetic experiments
were carried out under pseudo-first order conditions lI Sing a large excess of reagent. The complexes under investigation are all air stable and no spec ial precautions
were adopted for their kinetic runs.
CHAKRAVARTY el at.: BRIDGE CLEAVAGE REACTIONS OF Ru(lII) COMPLEXES
0·55
0·50
0·45
0·40
0·35
0·30
<II
u
l-
c
0
0·25
.D
....
0
UI
.D
0 ·20
4:
0·15
0-·10
Wove length,nm
Fig. 1 -Spectral scanning of the chl oride bridged complex (temp Sry)
[complex] = 7.5 x 10--4 mol dm-l; [HCI04] = O.S mol dm-3
(i) 0 min , (ii ) I h , (ii i) 2 h, (iv) 2.S h, (v) 3 h and (vi) 4 h.
Results and Discussion
D is soc iat io n
of
th e
trin uclea r
sp ec ies
fR u3(NH 3 ) /3 (SCN) Cl]Jf>+ in perchloric acid medium :
Absorption spectra and reaction stoichiometry .
The absorpti on spectra of the ch loro compl ex in aqueous perchloric acid solution is given in Fig. l . Curve (i)
shows two absorption maxima in the visible range at 488
nm and around 360 nm. In th e presence of perchl oric
ac id (0.5 mol dm-3) there is a little shift in max ima position, (from 495 nm to 488 nm) most probabl y due to
protonation of the complex species. Absorbance at 488
nm decreases continuously fo r nearly two hours wi th ou t
any change in the spectral pattern . However, after two
hours decrease in absorbance at 488 nm is very little,
but at 400 nm absorbance inc reases sign ificantly. Rate
cons tants measured at thi s wave length at the later period (after 2 hours) are different from those obtained at
687
the beginning of the reaction. It is likely that the reaction enters into a new phase at this stage. We have measured the rate constant at this wave length also to determine the rate constant of the possible reaction taking
place at this stage.
Dissociation of the trinuclear complex at different
bridge bonds will lead to a number of pentaammine and
tetraammine products. Tetraammine complexes may
exist in cis-trans isomeric forms and the thiocyanato
species in linkage isomeric forms. Dinuclear complex
species may also form in the course of the reaction.
Near the end of the first reaction (around ninety minutes after the start of the reaction), the reaction was arrested by cooling the reaction mixture to room temperature ( -200 ) and neutralizing the remaining acid with
KHCO r After cooling to ice temperature and separating deposited KCI0 4, the reaction mixture was loaded
on to a column of cation exchange resin (15 cm length,
Biorad AG 50W-X2), in the Na+ form, and eluted first
with water and then with acidic sodium perchlorate solution (PH-3.D). Concentration of sodium perchlorate
in the eluent was gradually increased from an initial 0.2
mol dm-3 to a final 3.0 mol dm-3 . Aliquots of 10 cm3
each were collected and their UV-visible spectra were
recorded. Four different complex species were detected
in the reaction mixture. Two of them are major constituents, [Ru(NH 3 )sNCSP+ (Amax in the visible range at 495
nm, difference from Ru-SCN complex having Amax in the
visible rang e pea k a t 505 nm 9 a nd trans [Ru(NH 3)/ HP )2]'+' having Amax at 332 nm. These two
components separated with the eluents 1.2 and 2.6 mol
dm-3 NaCI0 4 . Presence of two other minor components
ha ve a lso been de te c te d . Th ese a re trans [Ru(NH)/HP)CIF+and trans - [Ru(NH)4CI2]+(eluted
with 0.5 and 1.8 mol dm-3 NaCI04). These two species
have been identifi ed by matching their spectra with authentic samples 10. I I.
The colour of the reacti on mi xture, after subs ta nti al
progress of the second stage intensified (deep violet) and
after 2-3 days a bl ack residue was deposited. This residue was insoluble in all solvents and seems to be polymeric in nature. No attempt was made to determi ne its
composition.
D is socia tion
of
th e
[Ru./NH ) jSCN)Br)f>+
Trinuclear
sp ecies
Di ssociation of this compound could not be investigated in perchl oric ac id solu tion as the perchl orate salt
of the compound is insoluble in water and quic kly set-
688
INDIAN J CHEM. SEC A, JULY 1999
50W-X2, 15 cm column) and eluted with acidified (PH
- 3.0) sodium nitrate solution. Concentration of sodium
nitrate in the eluent was gradually increased from 0.1 to
3.0 mol dm-', Aliquots of 10 cm3 each were collected
and their UV-visible spectra were recorded. In the case
of the bromo complex also two major species were
identified in the solution as [Ru(NH3)5NCS] (NO , ),• and
trans - [Ru(NH')4Br2]NO,. Identification was made by
matching their respective spectra with known
samples 9 . '2.Trace amount of other species
[Ru(NH)4 Br(HP)F+ and trans - [Ru(NH, )iHP)2P+
were also detected in the mixture, the first one by equilibrating with a 0.1 mol drn-3 sodium bromide solution
and matching the spectra with that of the dibromo complex as before and the diaqua complex as earlier ill . These
species were eluting out with 1.2, 1.6, 0.5 and 2.4 mol
dm- 3 sodium nitrate solutions respectively.
CII
u
C
o
.0
'-
o
VI
.0
ci
Kinetics of the dissociation of chloride bridged complex
370
530
570
610
Wove length, nm
Fig.2 -Spectral scanning of the chloride bridged com plex in presence of nitric acid (temp 59") [complex] = 7.5 x 10-4 mol
dm-l; [HNO)l = 0.5 mol drn-) (i) 0 min, (ii) 15 min, (iii) 30
min , (iv) 45 min , (v) I h, (vi) 2 h, (vii) 3 h, (viii) 4 h.
tIes as an insoluble residue. So the study of the present
complex was made in nitric acid_ The spectral scanning
of the reaction mixture shows some initial (for ca. 30
minutes) rapid change in absorbance at 488 nm and thereafter slow increase in absorbance at 412 nm Fig. 2. After 2-3 hours the colour of the solution intensified (violet-red) and on concentration on a rotary evaporator some
sparingly soluble material deposited at the bottom of the
vessel. The remaining solution on cooling in a refrigerator overnight after adding a little (20 percent) alcohol
deposited crystals of [Ru(NH,)/HP)2P+' The sparingly
so luble material was identified as trans - [Ru(NH,)4Br2]
NO, by elemental analysis and also by matching its spectra with the authentic sample'~. The diaquo complex was
identified from its elemental analysis. Trans - structure
seems logical for it in parity with other products and
also from the fact that all such ruthenium (III) complexes
are stereoretetive" '.
As in the case of the chloro complex, during the course
of the dissoc iation reaction (about one hour from the
beginning) the reaction was arrested by cooling the reaction mixture to room temperature (-20°), excess acid
was neutralised with sodium bicarbonate and then loaded
on to a column of cation exc hange resin (Bio-rad AG
Kinetics of the acid catalysed dissoc iation of the
trinuclear chloride complex was followed at 59°. Rate
constants were determined by Guggenheims plot at 400
nm (Table I). Plot of rate constant versus [acid] for the
first stage of the reaction is linear having a slope and an
intercept on rate axis. However, for the second stage of
the reaction, plot of rate constant versus [H+] is li near
with posi tive slope, but no intercept. All the slopes and
intercepts reported in the Table I are determined by least
squares method and the observed deviations are standard deviation. The nature of acid dependence of the reaction gives the empirical rate equations (I, 2).
First stage,
... ( I )
Second stage,
.. . (2)
Rate constant values of these acid-dependent kll
and k21 and acid-independent (kill) paths are given in
Table 2.
It has been amply demonstrated that ruthenium(III),
and also ruthenium(II), indicate a more associative
character for water exchange and other substitution reactions(,' ". In aqueous solution, incoming aqua li gand
may form a seven-coordinated aqua intermediate with
any of the ruthenium atoms. However, it is difficult to
dislodge coordinated ammonia ligand because of strong
metal-nitrogen bond . Approach of the aqua ligand to first
ruthenium atom would lead to the formati o n of
aquapentaammine complex . However, this species could
not be detected in the reaction mixture. Hence, it is very
CHAKRAVARTY e/ at.: BRIDGE CLEAVAGE REACTIONS OF Ru (lII) COMPLEXES
likely that the first ruthenium atom does not form aqua
intermediate with the incoming water molecule. With
remaining two ruthenium centres, incoming water molecule may replace either of the chloride ligands, bridging or terminal, or the bridging thiocyanate ion. Thiocyanate is strongly bonded to two ruthenium centres and
forms a stronger bridge than chloride ion. The bridging
chloride ion is under a greater strain than th e terminally
coordinated chloride ion.
Approac h of the incoming water molecule may be
directed to any of the ruthenium atoms, but it is the middle one that form s the intermediate with greater ease than
others. Formation of the aqua intermediate by the middle ruthenium atom will lead to chloride bridge bond
breaking between the second and the third rutheniun atoms. Nevertheless, we were unabl e to detect the dinuclear
species with NCS- bridge. Such bridged species forms
only with ruth eniu m (II)~. As soon as the chloride bridge
breaks down, rupture of the SCN- bridge also takes place
giv in g the products. Thiocyanate io n, which is an
ambidentate li ga nd , is linked to two adjacent ruthenium
atoms through Sand N donor atoms. Ruth enium (III) is
a moderately hard acid; so Ru-N bond is stron ge r than
Ru-S bond. Dissociation of the complex takes place by
the breaking of Ru -S bond. Product analysi s indicated
formation of (85 ± 10)% Ru (NH), NCS2+ species. Traces
of su lphur bonded species might be there but of neg l igible sign ificance.
Attack of RO+ (or H,O) on the central ruthenium atom
.'
takes place in the direction of one of the C 2 axis which is
perpendicular to the C 4 axis of the molecular ion, as thi s
direction is least blocked by the ligands.
The ac id-independent and acid-dependent paths of the
co mplex ion may be shown as :
689
In the experimental temperature and acidity range both
the chloroaquo and the dichlorotetraammine complexes
undergo aquation to diaquo complex III . The products
obtained with the second and the third ruthenium atoms
would be trans isomers only since the trinucJear complex has a linear structure. It has been observed that during a substitution reaction , cis-tetramine complexes of
ruthenium(III) retain their configuration without undergoing conversion lO . In the present substitution reaction
if any cis species is formed at any stage of the reaction,
it would remain as cis throughout the course of the reaction . However, product diaquo species has a trans structure; so only trans species is formed in the course of the
reaction. In the presence of perchloric acid , it has been
observed earlier that these tran s products undergo dissociation and oxidation by the released coordinated ammonia molecul es with the formation of aquonitroso complexes l4 • These aquonitroso complexes undergo po lymerization 14 as observed in the second stage of th e reaction. The acid dependent path may also be shown in a
s imilar way as the acid-independent path. In this case a
hydronium ion (H jO+) approaches the central (second in
order) ruthenium atom. The products are the same, with
central ruthenium atom giving a protonated species.
The second stage of the reaction is primarily a polymerization process. Polymerization may take place "vith
the aquonitroso complex or its aquation or dissociati on
products, probably with more coordinated aqua and chl oride ions . This type or polymerization has been observed
with a number of ruthenium(III) complexes havin g coordi nated aqua and chloride ions I.'. Composition of thi s
polymer could not be determined with certainty. 1t is difficult to predict the mechanism of this polymerization
reaction at this stage, but it has been observed earl ier
that in the presence of perchlorate ion photochemical
oxidation of aqua complexes of ruthenium (III) leads to
formation of polymeric products l (, .
kif)
~
-Ru-NCS-Ru-CI-RlI-Ci + Hp
( I)
(3)
slow
-Ru -NCS ---- Ru ---- CI -
~
m
fast
I
(I)
:
(2)
Ru (3)
CI
Hp
Hp
(N H)5 RlI SCN l> + Ru(NH).(HP)/+ + RlI (N HJ)} HP)Cll+
( I)
(2)
(2, 3)
+ Ru(NH J).Cl 1 •
(3)
Dissociation o{ the bromide and thiocyanate bridged
complex
The rate plot of this complex shows that durin g dissoc iation two consecutive reactions are taking place. The
rate constants of these reactions were determined us ing
Guggenheims procedure by measing absorption at 488
and 412 nm respectively. The rate constant values thus
obtained are given in Table I. Plot of rate constant versus [acid] for both the early and the later period reactions are linear with positive slopes but only positive
intercept for the first one. The second plot has no
690
INDIAN J CHEM. SEC A, JULY 1999
Table I - Dissociation of [Ru)( NH ) ,)(SCN)X 2] 6+ in acidic aqueous solution
[Complex] = 5
X
10-4 mol dm-); 1= 1.0 mol dm-); temp. 59" (Error in k values is within
100k,
0.5
S·I
X
Acid
[W] mol dm-)
0.3
CI -
HCIO.
kI
4.86
6.0 1
7.3 1
HCIO.
kl
2.35
3.80
5.00
HNO)
k,
5.02
5.8 8
7.41
HNO)
k)
2.64
3. 10
3.61
HNO)
k.
1.10
1.90
2.71
Br-
± 3%)
0.7
Table 2 - Rate constants for ac id independent and acid dependent paths (x I (1) at 59"C
Acid independent
Complex
Acid dependent, k(dm)mol-'s- ' )
( S- I)
(First stage)
(Second stage)
(k ll ) 7.35
Chloro (HCIO)
(kill) 3.48
± 0.26
(k ,,) 5. 15 ± 0.23
Chloro (HNO)
(k lO ) 3.52 ± 0.22
(k ,, ) 5.06 ± 0.28
Bromo (HNO)
(k),) 1.90 ± 0.1 3
(k )2 ) 2.43 ± 0.10
intercept. Considering the nature of acid-dependence,
the empirical rate equations may be given as in Eqs (3
and 4) .
First stage,
k
Second stage,
k
IB,
(lIhslll
(nh.~ J I
lO r
=
=
k)(, + k),[W]
... (3)
k41[W]
...(4)
The rate constant values of the acid-catalysed k 11 and
and acid free k,o paths for this complex are also given
in Table 2.
Product analysis of the reaction mixture after nearly
thirty minutes indicates that both the bromide and the
ch loride bridged complexes dissociate in similar fashion, that is, by the approach of the incoming Hp or Hp+
li gand towards the central ruthenium atom with ultimate
bond cleavage between this ruthenium atom and the
bridging thiocyanate ion and also by the cleavage of the
bond between the third ruthenium atom and the bromide
bridge in a fast reaction step. Rate constants are smaller
compared to the chloro complex, may be due to higher
enthalpy necessary for metal-bromide bond rupture. In
halopentaammi ne comp lexes of cobalt(III),
chromium(III), rhodium(III) and iridium(III) it has been
k41
(First stage)
± 0.28
(k. 1 ) 4.02 ± 0.28
observed earlier that the metal -bromide bond rupture
requires a higher enthalpy than the metal-chloride bond
rupture 17. Bond making by the incoming aqua ligand is
more important in the case of the ruthenium(lH) complex than in cobalt(III) complexes. Bromide ion is comparatively a more soft base than the chloride ion. It should
lead to destabilization of the transition state in an SN2
process and consequently slower reactions with higher
activation energy may be observed. It is long known that
halide ions (CI-, Br-, 1- apart from F-) retain some basicity (Brollsted) even after coordination to a metal ion I X.
In acid soluti on, protonation of these halide ion s produce a reactive protonated species that hydrolytically
cleaves more easily. Chloride ion coordinated to ruthenium ion retains significant basicity' Y, and as both the
c hl oride and the bromide ions differ little in th e ir
Bronsted bas icity, protonation of bromide ion in thi s case
a lso produces a spec ies that cleaves more eas il y.
Aquation of the aquobromo complex formed after first
stage of the reaction may be shown as follows :
CHAKRAVARTY et al.: BRIDGE CLEAVAGE REACTIONS OF Ru(III) COMPLEXES
k4
References
[Ru(NH)).(Hp)(HBr)p+ + Hp -) [Ru(NH).(HP)Y+ + HBr ... (6)
slow
This mechanism is in conformity with the empirical
rate equation proposed in (4). The thiocyanato
pentaammine complex formed during the reaction does
not undergo aquation at this experimental condition. It
was
observed
earlier
that
at
59" the
pentaamminethiocyanato complex does not hydrolyse
even in the presence of 1.0 mol dm-3 PTS 20. Cobalt
pentaammine complex also shows a similar trend where
the thiocyanato complex indicates a greater inertness and
under similar conditions its rate of reaction is at least
104 times less than either of the chloride or the bromide
complexes 21 •
Final reaction products for the chloro and the bromo
complexes are different as the investigations were carried out for these two complexes in two different acidic
medium, viz. perchloric and nitric acids . In order to compare the reactions of these complexes, the kinetics of the
chloro-complex was also investigated in HNO, medium .
Spectral scanning of this complex in 0 .5 mol dm-3 HN0 3
at room temperature (ca. 25") shows similar behavior as
with the HCI0 4 with absorption maxima at 488 nm (first
stage) and 400 nm (second stage). The rate constants for
the first stage as determined by Guggenheim's procedure at 59" are given in Table I . Within the limits of
experimental error the values are the same as obtained
in HCI0 4 . No attempt was made to determine the rate
constant for the second stage of the reaction . The final
product in thi s case is not a polymeric one, but the diaquo
complex [Ru(NH) 4(HP)2P+' as obtained with the bromo
complex. So, the mechanisms of the reaction of both th e
complexes in nitric acid medium are similar.
691
Springborg J, Adv illorg Chem, 32 (1989) 560; Sykes A G,
References given in Inorgan ic reaction mechanislIls, 5/Jecialists periodical reports, 1-7 (The Chemical Society,
London) 1969-1979.
2
Richardson DE, Sen J P, Buhr J D & Taube H, In O/~~ Chem,
21 ( 1982) 3136.
3
Femi Iyun J & Musa K Y, Indian J Chem, 35A ( 1996) 210.
4
Taquikhan M M & Naik R M, J mol Catal 54 ( 1989) 139.
5
Benjamin J C, Meyer T J & White P S, Inorg Ch elll 34
( 1995) 3600; Siddique S, Hendersen W W & Shephered
R E, Inorg Chem, 26 ( 1987) 3 101 .
6
Poulopoulou V G & Taube H, In org Chem, 36 (1997) 4782.
7
Naomi G, Nallas A, Jones S W & Brewer K J, In O/g Ch elll ,
35 ( 1996) 6974.
8
Prasad R, Yadav S K S & Agarwalla U, J inorg I//lel Chelll.
43 (1981) 2359.
Yadav S K S & Agarwalla U, Polyhedron, 3 (1984) I.
Poon C K & Isabirye D A, J chem Soc Dalt, ( 1977) 2115 .
Kohata S & Ohyashi A, Illorg chim Acta, 64 ( 1982) L 119.
Glue K & Breul W, Z al/org Chem, 237 (1938) 326.
Cleary D H. Helm L & Merbach A E, JAm chelll Soc 109
( 1987) 4444 ; Hugi A D, Helm L & Merbach A E. II/ org
Chem. 26 ( 1987) 1763.
Taube H. II/org Chel11, 25 ( 1986) 33 18; Winkler J R. .I Alii
chem Soc, 109 ( 1987) 2381 ; Furholy U & Haim A, Illorg
Chel11, 26 (1987) 3243.
Taquikhan M M, Ramchandraiah G & Sukla R S, Polrh e
drol/, II ( 1992)3075.
Durham B, Wilson S R, Hogdson D J & Meyer T J, .I Alii
chem Soc, 102 ( 1980) 600.
Lamb A B, .1 Am chem Soc, 61 (1939) 699 ; Adamson A w
& Basolo F, Acta chem Sccmd, 9 ( 1955) 1261 .
Swaddle T W & King E L, II/org Chem, 4 ( 1965) 532.
Huang D. Folting K & Caulton K G, II/ org Chelll . 35
( 1996) 7035.
Adhikari S, Ph.D thesis (The University of Kalyani ) 1990.
Gay 0 L & Lalor G C, J chelll Soc, ( 1966) A 1179.
I
9
10
II
12
13
14
15
16
17
18
19
20
21