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Acta Crystallographica Section B

Structural
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ISSN 0108-7681

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First bent form for the hydroxo-bridged cis-diammineplatinum(II) dimer[Pt2 (NH3 )4(-OH)2 ](ClO4)2Ken Sakai, Yosuke Konno, Noboru Takayama and Satoru Takahashi

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Acta Cryst. (2004). B60, 255–262

Ken Sakai et al.  Bent form for cis -diammineplatinum(II)research papers
Acta Crystallographica Section B

Structural
Science
ISSN 0108-7681

Ken Sakai,* Yosuke Konno,
Noboru Takayama and Satoru
Takahashi
Department of Applied Chemistry, Faculty of
Science, Tokyo University of Science, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-8601, Japan

Correspondence e-mail:ksakai@rs.kagu.tus.ac.jp

First bent form for the hydroxo-bridged
cis-diammineplatinum(II) dimer[Pt2(NH3)4(l-OH)2](ClO4)2
The third crystal structure containing the hydroxo-bridged cisdiammineplatinum(II) dimer has been determined for aperchlorate salt of the complex, [Pt2(NH3)4(-OH)2](ClO4)2.
However, the dinuclear cations in the nitrate and thecarbonate salts, [Pt2(NH3)4(-OH)2](NO3)2 [Faggiani,
Lippert, Lock & Rosenberg (1977). J. Am. Chem. Soc. 99,
777±781] and [Pt2(NH3)4(-OH)2](CO3)H2O [Lippert, Lock,
Rosenberg & Zvagulis (1978). Inorg. Chem. 17, 2971ÿ2975],
were reported to possess a nearly planar geometry. The cationin the title perchlorate salt has been found to possess an
exceptional bent form in which two Pt coordination planes
within the dimer are tilted at an angle of 151.7 (1) to one
another. The diplatinum entity has a syn orientation withregard to the conformation of two hydroxo bridges, in part due
to the one-dimensional hydrogen-bonding network achievedin the crystal structure. DFT MO investigations have also been
carried out to reveal that the planar-bent selection could be
induced by the anti±syn selection at the H(hydroxo) atoms.
Comparison has also been made between the geometricalfeatures of the three salts from the viewpoint of theorientation of H(hydroxo) atoms.

Received 3 December 2003
Accepted 26 February 2004

1. Introduction# 2004 International Union of Crystallography
Printed in Great Britain ± all rights reserved

Acta Cryst. (2004). B60, 255±262

We have thus far succeeded in the development of `dimerbased' one-dimensional platinum chain compounds (Sakai etal., 1998, 2002). In these studies homo-valent and mixed-valentcarboxylate-bridged cis-diammineplatinum dimers [Pt2(NH3)4(-carboxylato)2]2+ (carboxylate = acetate, propionateetc.) have been found to afford one-dimensional platinum
chain structures. In these crystal structures the dimer±dimer
associations are stabilized with quadruple hydrogen bonds aswell as metal±metal bonds formed between the dinuclear
entities that are stacked in a one-dimensional fashion. During
the course of these studies we found that the diplatinum unit
in the title compound, [Pt2(NH3)4(-OH)2](ClO4)2 (I), gives a
related one-dimensional framework primarily based on thehydrogen-bonding associations in the crystal structure. Moreinterestingly, it is also found that the diplatinum cation
involved in (I) has an exceptional `bent' structure. Previously,
a nitrate and a carbonate salt of the complex, [Pt2(NH3)4(OH)2](NO3)2 [abbreviated as (II); Faggiani et al., 1977] and[Pt2(NH3)4(-OH)2](CO3)2H2O [abbreviated as (III);
Lippert et al., 1978], have been structurally determined by Xray diffraction. In each salt, the diplatinum unit has been

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DOI: 10.1107/S0108768104004513

255

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ef®cient anticancer drug (Jamieson & Lippard, 1999).

Table 1

Experimental details for (I).
Crystal data
Chemical formula
Mr
Cell setting, space groupÊ)a, b, c (A
Ê 3)
V (A
Z
Dx (Mg mÿ3)
Radiation typeNo. of re¯ections forcell parameters
 range ( )
 (mmÿ1)
Temperature (K)Crystal form, colour
Crystal size (mm)Data collection
Diffractometer
Data collection method
Absorption correction
Tmin
Tmax
No. of measured, independent andobserved re¯ections
Criterion for observed re¯ections
Rintmax ( )Range of h, k, lRe®nementRe®nement on
R[F2 > 2(F2)], wR(F2), S
No. of re¯ections
No. of parameters
H-atom treatment
Weighting scheme(/)max
Ê ÿ3)max, min (e A
Extinction method
Extinction coef®cient
Absolute structure
Flack parameter

Cl2H14N4O10Pt2
691.23
Orthorhombic, P212121
7.3422 (4), 12.9876 (7), 13.9429 (8)1329.56 (13)
4
3.453
Mo K
4531
2.9±28.1
21.47
296 (2)
Needle, pale yellow
0.2  0.10  0.06
Bruker SMART APEX CCD area
detector diffractometer! scans
Multi-scan (based on symmetryrelated measurements)(SADABS; Sheldrick, 1996)
0.069
0.264
7867, 2898, 2726
I > 2(I)
0.040
27.1
ÿ9 ) h ) 9ÿ16 ) k ) 12
ÿ15 ) l ) 17
F2
0.024, 0.056, 0.98
2898
168
Constrained to parent site
w = 1/[ 2(Fo2)]<0.0001
1.21, ÿ1.23
SHELXL
0.00081 (8)Flack (1983), 1361 Friedel pairs
0.012 (12)Computer programs: SMART (Bruker, 2001), SAINT (Bruker, 2001), SHELXS97(Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), KENX (Sakai, 2002), TEXSAN(Molecular Structure Corporation, 2001), ORTEPII (Johnson, 1976).

ascertained to possess a planar geometry. Here we report the
crystal structure of (I) as the ®rst experimental evidence
revealing the existence of a bent form for the [Pt2(NH3)4(OH)2]2+ cation. In addition, we report the results of DFT
calculations which have been carried out to better understandthe conformational properties of this compound. It should also
be noted that the present study must be viewed as related tothe theoretical investigations of AulloÈn et al. (1998, 2000), in
which planar-bent isomerism on the related single-atombridged dinuclear complexes were reported. Moreover, it mustalso be emphasized that the [Pt2(NH3)4(-OH)2]2+ dimer is
one of the possible reaction products in aqueous media of cisPtCl2(NH3)2 (cisplatin or cis-DDP), which is well known as an

256

Ken Sakai et al.  Bent form for cis -diammineplatinum(II)

2. Experimental
2.1. Synthesis of [Pt2(NH3)4(l-OH)2](ClO4)2

A solution of cis-PtCl2(NH3)2 (1 mmol) and AgClO4(2 mmol) in H2O (7 ml) was stirred at 343 K for 3 h in thedark, followed by ®ltration to remove the AgCl precipitated.
The pH of the ®ltrate was then adjusted to 4.0 using aqueous
0.5 M NaOH solution. The resulting solution was heated at
313 K for 2 h without stirring. Allowing the solution to standat 278 K for 1±2 d afforded (I) as pale yellow needles (yield25%). Analysis: calculated for Cl2H14N4O10Pt2: H 2.04, N 8.11;
found: H 1.84, N 7.61%.
2.2. X-ray crystallography

A diffraction-quality single crystal of (I) was mounted on a
glass ®bre. Diffraction data were measured on a Bruker
SMART APEX CCD area-detector diffractometer with
Ê ). Agraphite-monochromated Mo K radiation ( = 0.71073 A
hemisphere of data was collected in 10 s frames using ! scansof 0.3 per frame. Data reduction was performed with SAINT(Bruker, 2001), which inter alia applies corrections for Lorentz
and polarization effects. Absorption corrections were applied
using SADABS (Sheldrick, 1996) and the space-group possibilities determined using XPREP in SAINT (Bruker, 2001).
Other details of unit-cell dimensions, data collection andre®nement are summarized in Table 1, together with details of
the software employed.1
The intensity statistics suggested a non-centrosymmetricspace group for (I) and P212121 was assigned on the basis of
the systematic absences. The absolute structure was determined from the value of the re®ned Flack (1983) parameter.
H(ammine) atoms were located at their idealized positions
Ê and Uiso(H) =and re®ned as riding atoms with NÐH 0.89 A
1.5Ueq(N). H(hydroxo) atoms were not located. In the ®nal
difference-Fourier synthesis only one residual peak aboveÊ from Pt1. The deepest holeÊ ÿ3 was observed, at 0.64 A
1eA
Ê
was located 0.95 A from Pt1.
2.3. DFT calculations

In order to better understand the conformational propertiesof the Pt2(-OH)2 core, we used the density functional theory(DFT) method implemented in the Gaussian98 suite of1

Supplementary data for this paper are available from the IUCr electronicarchives (Reference: BM5007). Services for accessing these data are describedat the back of the journal.

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Acta Cryst. (2004). B60, 255±262

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programs (Frisch et al., 1998). All calculations were carried out
using the B3LYP method which uses hybrid Becke's threeparameter exchange functional (Becke, 1993) with the correlation energy functional of Lee et al. (1988). Calculations were
performed using the standard double- type LanL2DZ basis
set (Dunning & Hay, 1976; Hay & Wadt, 1985a; Wadt & Hay,
1985) implemented in Gaussian98, without any additionalpolarization or diffuse functions. The LanL2DZ basis set alsouses relativistic effective core potentials (RECP) for the Pt
atoms to account for the scalar relativistic effects of the inner
60 core electrons, [Kr]4d104f14 (Hay & Wadt, 1985b).
As previously reported by AulloÈn et al. (2000), ®ve possible
conformers illustrated in Fig. 1 were constructed as the initialstructures for the geometry optimization studies. The initialstructures for the planar forms with an anti and a syn con®guration (pa and ps in Fig. 1) were based on the crystal structure of the nitrate salt [Pt2(NH3)4(-OH)2](NO3)2 (II)(Faggiani et al., 1977). The initial structures for three different
bent forms (ba, be and bx in Fig. 1) were constructed based on
the crystal structure of the title perchlorate salt [Pt2(NH3)4(OH)2](ClO4)2 (I). Structures were fully optimized without
applying any structural restraints.

shifted toward the upper side of a mean plane de®ned by eight
non-H atoms within the dinuclear cation. This indicates thatthe two H(hydroxo) atoms, whose locations were not clearly
determined, must be oriented upward in Fig. 2(b) so that they
can achieve appropriate hydrogen-bonding interactions withthe perchlorate O atoms. This conformation corresponds tothe be form de®ned in Fig. 1, an assignment which will befurther supported by the interionic hydrogen-bonding
geometry described below. The most remarkable feature isthat the two Pt coordination planes within the dinuclear cation
are inclined by 151.7 (1) to one another (see Fig. 2c). In (II)and (III) the diplatinum entities have a nearly planar
geometry [180 for (II), see also Fig. 3; 176.8 (5) for (III), seealso Fig. 4]. As a result, the intracation Pt  Pt distance in (I)Ê ] is ca 0.05 AÊ shorter than those in (II)[Pt1  Pt2 3.0421 (4) A

3. Results and discussion
3.1. Description of the structure of (I)As shown in Fig. 2, a dinuclear cation and two perchlorateanions comprise the asymmetric unit of (I). Selected bonddistances and angles are summarized in Table 2. Although thecoordinate bond distances in (I) are all similar to thosereported for (II) and (III), the corresponding valence angles in(I) are slightly different owing to the bent nature of thecomplex (see Table 2). Two perchlorate anions are associatedwith the dimer cation through the hydrogen bonds formed
between the O atoms of perchlorates and the hydroxo/ammine
ligands [O1  O4 2.814 (9), O2  O8 2.891 (8) and N4  O9
Ê ]. As shown in Fig. 2(b), the two ClOÿ2.990 (9) A
4 ions are

Figure 2

Figure 1De®nition of ®ve possible conformers for the [Pt2(NH3)4(-OH)2]2+
cation, where all the structures are viewed along the Pt  Pt vector andthe ammines are omitted for clarity.
Acta Cryst. (2004). B60, 255±262(a) A top view, (b) a side view along the Pt  Pt vector and (c) a side view
perpendicular to the Pt  Pt vector for the contents of the asymmetricunit of (I), showing the atom-labelling scheme. Displacement ellipsoids
are shown at the 50% probability level. In (c) the perchlorate anions areomitted for clarity.
Ken Sakai et al.  Bent form for cis -diammineplatinum(II)electronic reprint257

research papers
Table 2

Ê ,  ) of (I)±(III).
Comparison of the geometries (AThe crystallographic data together with the atom-labelling scheme are given in Fig. 3 for (II) and in Fig. 4 for(III). Values for (II) (Faggiani et al., 1977) and (III) (Lippert et al., 1978) are taken from the literature.
(I)

(II)

(III)

Pt  Pt(intradimer)

Pt1  Pt2 3.0421 (4)Pt  Pti 3.085 (1)Pt1  Pt2 3.104 (1)Pt  Pt(interdimer)

Pt1  Pt1ii 3.6968 (6)Pt  Ptiii 4.578 (1)Pt2  Pt2iv 3.167 (1)Pt1  Pt2v 3.443 (1)PtÐNPt1ÐN1 2.027 (7)Pt1ÐN2 2.029 (6)Pt2ÐN3 2.040 (6)Pt2ÐN4 2.018 (6)PtÐN2 2.02 (1)PtÐN1 2.01 (1)Pt1ÐN1 2.05 (2)Pt1ÐN2 2.02 (2)Pt2ÐN3 2.04 (2)Pt2ÐN4 2.02 (2)PtÐOPt1ÐO1 2.047 (5)Pt1ÐO2 2.046 (6)Pt2ÐO1 2.038 (6)Pt2ÐO2 2.053 (5)PtÐO1 2.03 (1)PtÐO1i 2.03 (1)Pt1ÐO1 2.02 (1)Pt1ÐO2 2.04 (1)Pt2ÐO1 2.07 (1)Pt2ÐO2 2.06 (1)NÐPtÐN

N1ÐPt1ÐN2 90.2 (3)N3ÐPt2ÐN4 90.4 (3)N2ÐPtÐN1 89.3 (6)N1ÐPt1ÐN2 88.0 (7)N3ÐPt2ÐN4 90.0 (7)NÐPtÐO in cis
positionsN1ÐPt1ÐO1 95.3 (2)N2ÐPt1ÐO2 95.1 (3)N3ÐPt2ÐO1 94.9 (3)N4ÐPt2ÐO2 95.1 (2)N2ÐPtÐO1 94.9 (5)N1ÐPtÐO1i 94.5 (5)N1ÐPt1ÐO1 97.5 (6)N2ÐPt1ÐO2 92.6 (6)N3ÐPt2ÐO1 95.6 (6)N4ÐPt2ÐO2 93.5 (6)NÐPtÐO in transpositionsN1ÐPt1ÐO2 174.4 (2)N2ÐPt1ÐO1 174.0 (3)N3ÐPt2ÐO2 174.4 (3)N4ÐPt2ÐO1 174.0 (3)N2ÐPtÐO1i 176.1 (6)N1ÐPtÐO1 175.6 (6)N1ÐPt1ÐO2 175.9 (6)N2ÐPt1ÐO1 174.5 (6)N3ÐPt2ÐO2 175.6 (5)N4ÐPt2ÐO1 170.5 (4)OÐPtÐOO1ÐPt1ÐO2 79.5 (2)O1ÐPt2ÐO2 79.5 (2)O1ÐPtÐO1i 81.3 (4)O1ÐPt1ÐO2 81.9 (5)O1ÐPt2ÐO2 80.6 (5)PtÐOÐPt

Pt1ÐO1ÐPt2 96.3 (2)Pt1ÐO2ÐPt2 95.8 (2)PtÐO1ÐPti 98.6²

Pt1ÐO1ÐPt2 98.8 (5)Pt1ÐO2ÐPt2 98.5 (5)Symmetry codes: (i) 1 ÿ x; 1 ÿ y; ÿz; (ii) 12  x; 12 ÿ y; ÿz; (iii) ÿx; 1 ÿ y; 1 ÿ z; (iv) 1 ÿ x; ÿy; 1 ÿ z; (v)ÿx; ÿy; 1 ÿ z. ² This value was not reported in the original paper (Faggiani et al., 1977), and was therefore calculated
in TEXSAN (Molecular Structure Corporation, 2001).

Ê ] or (III) [3.104 (1) A
Ê ] for (III). Additional[3.085 (1) Afeatures are the smaller OÐPtÐO/PtÐOÐPt angles and thelarger NÐPtÐN angles in (I) compared with the corresponding angles in (II) and (III) (see Table 2).
The dinuclear cations are related by the operation of a 21
screw axis and stack along the a axis to give a pseudo-onedimensional platinum chain (see Fig. 5). Although the interÊ ; (i)dimer Pt  Pt distance [Pt1  Pt1i 3.6968 (6) A1
1
x  2 ; ÿy  2 ; ÿz] is relatively long, we conclude that a weakmetal±metal interaction is, to some extent, promoted between
the PtII centres, since the colours of compounds (I)±(III) seemto be correlated with their shortest interdimer Pt  Pt
Ê for the colourlessdistances: Pt  Pt(interdimer) 4.578 (1) AÊ for thenitrate salt (Fig. 3) > Pt  Pt(interdimer) 3.6968 (6) AÊpale yellow perchlorate salt > Pt  Pt(interdimer) 3.167 (1) A
for the deep yellow carbonate salt (Fig. 4). The dimer±dimer
association in (I) is also stabilized with two hydrogen bonds
formed between the ammines and the O atoms of hydroxoÊ ; (i)bridges [N1  O2i 2.947 (8) and N2  O1ii 2.973 (9) Ax  12 ; ÿy  12 ; ÿz; (ii) x  12 ; ÿy  12 ; ÿz; see the details in
Table 3]. These hydrogen-bonding geometries further support
the validity of the syn orientation of bridging hydroxo ligands

258

Ken Sakai et al.  Bent form for cis -diammineplatinum(II)in (I), and is considered as one of thereasons why the exceptional bent
form is stabilized in (I). As summarized in Table 3, the crystal packing isfurther stabilized with extensivehydrogen bonds formed between the
cations and the anions, leading to thethree-dimensional hydrogen-bonding
networks in the crystal structure.
Nevertheless, the O(hydroxo)Ð
H  O(ClO4ÿ) and N(ammine)Ð
H  O(hydroxo) hydrogen bonds are
effectively shorter and stronger thantheN(ammine)ÐH  O(ClO4ÿ)ones. In summary:(i) the dimer cation forms relatively
strong hydrogen bonds with the twoneighbouring perchlorate anions(Figs. 1a and b),(ii) these ion-pair aggregates form
moderate N(ammine)ÐH  O(hydroxo) hydrogen bonds with one
another to give a one-dimensional
chain in Fig. 5 and(iii) the interchain interactions
comprise relatively weak N(amhydrogenmine)ÐH  O(ClO4ÿ)bonds.

3.2. DFT investigations

Structures of the initial and the
optimized geometries given for the

Figure 3

The stair-like dimer chain observed in [Pt2(NH3)4(-OH)2](NO3)2 (II),where the crystal data are taken from the literature [Faggiani et al., 1977;Ê, a = 6.763 (12), b = 7.890 (18), c = 7.256 (13) A
triclinic, space group P1,= 92.3 (1), = 133.1 (1), = 91.0 (1) , Z = 1]. H atoms and nitrate anions
are omitted for clarity. Dashed lines denote hydrogen bonds.

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Table 3Ê ,  ) for (I).
Hydrogen-bonding geometry (ADÐH  A
O1  O4²
O2  O8²
N1ÐH1A  O2i
N2ÐH2A  O1i
N2ÐH2B  O5ii
N4ÐH4A  O6iii
N1ÐH1C  O8ii
N4ÐH4B  O9
N3ÐH3A  O9iv

DÐH

0.89
0.89
0.89
0.89
0.89
0.89
0.89

H  AD  A

DÐH  A

2.10
2.09
2.24
2.32
2.44
2.10
2.46

2.814 (9)2.891 (8)2.947 (8)2.973 (9)3.127 (11)3.178 (9)3.044 (8)2.990 (9)3.007 (9)

158.4
170.7
172.0
161.5
125.3
179.1
120.1

Symmetry codes: (i) 12  x; 12 ÿ y; ÿz; (ii) x ÿ 12 ; 12 ÿ y; ÿz; (iii) 32 ÿ x; ÿy; 12  z; (iv)3
1
² H atoms on the hydroxyl ligands are not located.
2 ÿ x; ÿy; z ÿ 2.

®ve test cases are shown in Fig. 6. The SCF (self-consistent®eld) energies obtained for the ®ve optimized geometries are
summarized in Table 4. Selected interatomic distances andangles given for the optimized structures are also listed inTable 5. As shown in Fig. 6, geometry optimizations startingfrom the pa and ba conformers both gave the pa conformer as
the energy-minimized structure, indicating that the planar
skeleton is favourable when the H(hydroxo) atoms are locatedin an anti orientation. On the other hand, optimizations
starting from the ps, be and bx conformers all gave the samestructure corresponding to the bx conformer as the energyminimized structure, where the dihedral angle between the Pt
coordination planes within the dimer unit converged at
150.7 (1) . This is quite consistent with the value observed in(I) [151.7 (1) ], even though the dimer cation in (I) does nothave a bx form, but has a be form. Moreover, the latter threecalculations reveal that the bent skeleton is preferable when
the H(hydroxo) atoms are oriented in a syn fashion (note thateither endo or exo is considered as syn). The two energyminimized conformers are only slightly different in energy; theoptimized conformer bx is 14.57 kJ molÿ1 lower in energy
than the pa one (see Table 4), indicating that the bent structure is essentially more stable in comparison with the planarone, at least, in the gaseous state.
3.3. Consideration for the planar-bent isomerism of the
dimer cations in (II) and (III)The orientation of H(hydroxo) atoms were not well determined in the previous reports for (II) and (III) (Faggiani et al.,
1977; Lippert et al., 1978). However, the detailed investigations of their hydrogen-bonding interactions achieved in the
crystal structure provide a clear description of the conformation with regard to the H(hydroxo) atoms in (II) and (III), as
follows. As shown in Fig. 3 the dimer cations in (II) arearranged in a stair-like fashion to give a one-dimensionalhydrogen-bonded array in which the shortest interdimer
Ê . The hydrogen-bondingPt  Pt distance is 4.578 (1) A
geometry clearly shows that the dimer cation in (II) has an anti
orientation.
On the other hand, Fig. 4 shows that a dimer of dimers isachieved in the crystal structure of (III), in which two dimers
are correlated through an inversion centre, a single strong
Ê ] within a tetraPt  Pt interaction exists [Pt  Pt 3.167 (1) A
nuclear aggregate, and the dimer±dimer association is also

Figure 4

Pseudo-one-dimensional network of tetranuclear aggregates in[Pt2(NH3)4(-OH)2](CO3)2H2O (III), where the crystal data are takenfrom the literature [Lippert et al., 1978; monoclinic, space group P21/c, a =Ê , = 119.15 (1) , Z = 4]. H atoms
7.127 (2), b = 11.416 (3), c = 15.379 (4) A
and water molecules are omitted for clarity. Dashed lines denotehydrogen bonds.
Acta Cryst. (2004). B60, 255±262

Figure 5

One-dimensional dimer chain in (I) growing along the a axis, where thedashed lines denote hydrogen bonds and the counterions are omitted forclarity.
Ken Sakai et al.  Bent form for cis -diammineplatinum(II)electronic reprint259

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Table 4

Summary for the geometry optimizations starting from the ®ve possible conformers.
See experimental details in x2. Structures on the same calculations are shown in Fig. 6. Hinge angle: the tilt angle between the two Pt coordination planes within the
dinuclear unit. The values have been calculated using TEXSAN (Molecular Structure Corporation, 2001). Values in parentheses correspond to the standard
uncertainties estimated in the mean-plane calculations.
Initial conformation

Optimized conformation

Hinge angle ( )SCF energy (kJ molÿ1)

Relative difference in energy (kJ molÿ1)

pa
ps
ba
be
bx

pa
bx
pa
bx
bx

179.854 (3)150.714 (3)179.449 (3)150.656 (3)150.788 (3)ÿ1616853.599
ÿ1616868.166
ÿ1616853.598
ÿ1616868.166
ÿ1616868.166

14.57
0.0
14.57
0.0
0.0

Table 5
Optimized geometries afforded by the ®ve calculations listed in Table 4.
Distances and angles have been calculated using TEXSAN (Molecular Structure Corporation, 2001).
pa ! pa

ps ! bx

ba ! pa

be ! bx

bx ! bx

Pt1  Pt2
Pt1ÐN5
Pt1ÐN6Pt2ÐN7
Pt2ÐN8Pt1ÐO3
Pt1ÐO4Pt2ÐO3Pt2ÐO4

3.199
2.085
2.085
2.085
2.085
2.083
2.083
2.083
2.083

3.110
2.080
2.080
2.080
2.080
2.085
2.085
2.085
2.085

3.198
2.085
2.085
2.085
2.085
2.083
2.083
2.083
2.083

3.110
2.080
2.080
2.080
2.080
2.085
2.085
2.085
2.085

3.110
2.080
2.080
2.080
2.080
2.085
2.085
2.085
2.085

N5ÐPt1ÐN6
N7ÐPt2ÐN8
N5ÐPt1ÐO3
N6ÐPt1ÐO4
N7ÐPt2ÐO3
N8ÐPt2ÐO4
N5ÐPt1ÐO4
N6ÐPt1ÐO3
N7ÐPt2ÐO4
N8ÐPt2ÐO3
O3ÐPt1ÐO4
O3ÐPt2ÐO4
Pt1ÐO3ÐPt2
Pt1ÐO4ÐPt2

93.86
93.86
93.28
93.22
93.28
93.22
172.8
172.8
172.8
172.8
79.66
79.66
100.34100.34

95.12
95.20
92.29
92.24
92.23
92.22
172.1
172.0
172.0
172.0
80.22
80.22
96.47
96.45

93.88
93.87
93.17
93.31
93.18
93.31
172.7
172.9
172.7
172.9
79.67
79.67
100.33100.33

95.16
95.15
92.23
92.27
92.24
92.28
172.0
172.0
172.0
172.0
80.20
80.20
96.46
96.46

95.14
95.15
92.25
92.28
92.25
92.27
172.0
172.0
172.0
172.0
80.19
80.19
96.50
96.50

1.027±1.029
0.982
114.6±114.7
1211.027±1.029
0.981
113.8±114.1
1221.027±1.029
0.982
114.7
121

1.027±1.029
0.982
114.7
121

H-atom geometry given in a compact formatNÐH
1.027±1.029
OÐH
0.981
PtÐOÐH
113.9±114.0PtÐPtÐOÐH
122

supported by four hydrogen bonds formed between theammines and the O atoms of hydroxo bridges. The hydrogenbonding geometry clearly indicates that the H(hydroxo) atoms
are oriented towards the outside of the tetranuclear aggregate.
As shown in Fig. 4, two carbonate ions link the two neighbouring tetranuclear aggregates via relatively strong hydrogenbonds formed between the H(hydroxo) atoms and the O
atoms of carbonates [O(hydroxo)  O(carbonate) 2.86 (2)Ê ; Lippert et al., 1978]. These clearly show thatand 2.68 (2) A
the dimer cation in (III) has a syn orientation in spite of thenearly planar geometry.

4. Conclusions
We have for the ®rst time observed a bent form of the[Pt2(NH3)4(-OH)2]2+ cation in which the H(hydroxo) atoms

260

Ken Sakai et al.  Bent form for cis -diammineplatinum(II)have a syn orientation owing to the one-dimensionalhydrogen-bonding network. Our DFT investigations revealthat the planar-bent selection could be given by the anti±synselection at the H(hydroxo) atoms. Nevertheless, the carbonate salt shows that the syn orientation does not always lead tothe bent form of the diplatinum core. In conclusion, theconformation of the [Pt2(NH3)4(-OH)2]2+ cation is primarily
governed by the overall balance in the stability of crystal
packing.

This work was supported by Grants-in-Aid for Scienti®cResearch on Priority Areas (Nos. 10149248, 11136246 and12023247 `Metal-assembled Complexes') from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
This work was partly supported by a Grant-in-Aid for Scien-

electronic reprint

Acta Cryst. (2004). B60, 255±262

research papers

Figure 6

Structures of optimized geometries given from the ®ve possible initial structures: (a) pa ! pa; (b) ps ! bx; (c) ba ! pa; (d) be ! bx; (e) bx ! bx, where
the conformers before and after each arrow respectively correspond to the initial and the optimized structures in each optimization.

ti®c Research (B) (No. 14340223) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.

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