Buy article online - an online subscription or single-article purchase is required to access this article.
The title complex, (C
4H
12N
2)[Ni
2(C
6H
5O
7)
2(H
2O)
4], was synthesized under solvothermal conditions. Both cation and anion possess crystallographically imposed inversion symmetry. The citrate ion acts as a quadridentate ligand, coordinating through the hydroxyl and two carboxylate O atoms to one nickel atom, and bridging the second metal centre through the remaining carboxylate group. The coordination around each Ni
II atom is completed to distorted octahedral by the O atoms of two water molecules. The crystal structure is stabilized by intra- and intermolecular O—H
O and N—H
O hydrogen-bonding interactions.
Supporting information
CCDC reference: 650509
Key indicators
- Single-crystal X-ray study
- T = 298 K
- Mean (C-C) = 0.003 Å
- R factor = 0.029
- wR factor = 0.073
- Data-to-parameter ratio = 15.6
checkCIF/PLATON results
No syntax errors found
Alert level C
PLAT042_ALERT_1_C Calc. and Rep. MoietyFormula Strings Differ .... ?
PLAT232_ALERT_2_C Hirshfeld Test Diff (M-X) Ni1 - O1W .. 5.34 su
PLAT720_ALERT_4_C Number of Unusual/Non-Standard Label(s) ........ 4
0 ALERT level A = In general: serious problem
0 ALERT level B = Potentially serious problem
3 ALERT level C = Check and explain
0 ALERT level G = General alerts; check
1 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
1 ALERT type 2 Indicator that the structure model may be wrong or deficient
0 ALERT type 3 Indicator that the structure quality may be low
1 ALERT type 4 Improvement, methodology, query or suggestion
0 ALERT type 5 Informative message, check
Nickel chloride hexahydrate (0.072 g, 0.3 mmol) and citric acid monohydrate
(0.061 g, 0.3 mmol) were dissolved in water/ethanol (1:1 v/v)
solution (5 ml). Piperazine hexahydrate (0.096 g, 0.5 mmol) was then added and
the solution stirred for 30 min. The resulting solution was transferred into
Teflon-lined autoclave and heated at 130 °C under autogenous pressure for 5
days. Green block crystals suitable for X-ray analysis were collected from the
reaction mixture.
The structure was solved by Patterson method. All hydrogen atoms were included
in the riding model approximation, with C–H = 0.97 Å, N–H = 0.90 Å, O–H
= 0.85 Å, and with Uiso(H) = 1.2 Ueq(C, N, O).
Structure description
top
Up to now, hundreds of metal citrate complexes with diverse architectures have
been synthesized and well documented in the literature (Kaliva et al.,
2004; Kefalas et al., 2005; Wang et al., 2005; Xiang et
al., 2005; Zhang et al., 2006). Some complexes contain
centrosymmetric dimers with 1-D polymeric chain or 2-D layer structure (Zhou
et al., 2005; Baggio & Perec, 2004), some are similar to the title
complex (Baker et al., 1983; Kotsakis et al., 2003), most of
them have monovalent counter ions. In this paper, the complex we report has a
divalent organic piperazinium cation. Its structure is shown in Fig. 1. Each
citrate ligand is triply deprotonated, and chelates to the Ni atom through the
α-hydroxyl, α-carboxyl and one β-carboxyl oxygen atom. The other
β-carboxyl oxygen atom spans over to the second Ni atom of the dimer. The
distorted octahedral coordination sphere of each nickel atom is completed by
the oxygen atoms of two water molecules. Selected geometric parameters of the
complex are given in Table 1. The piperazinium cations occupy the space
between the nickel-citrate dimers. The anions and the cations are connected by
strong N—H···O hydrogen bonds. There are intramolecular hydrogen bonds
between the hydroxyl groups and the carboxyl groups. Hydrogen bonding
interactions are also observed between the coordinated water molecules and the
carboxyl groups of neighbouring anions, forming a three-dimensional network
(Table 2, Fig. 2).
For related literature, see: Baggio & Perec (2004); Baker et al. (1983);
Kaliva et al. (2004); Kefalas et al. (2005); Kotsakis et
al. (2003); Wang et al. (2005); Xiang et al. (2005); Zhang
et al. (2006); Zhou et al. (2005).
Data collection: RAPID-AUTO (Rigaku, 1998); cell refinement: RAPID-AUTO; data reduction: RAPID-AUTO; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL/PC (Sheldrick, 1993); software used to prepare material for publication: SHELXL97.
Piperazinium tetraaquabis(µ
2-citrato)dinickelate(II)
top
Crystal data top
(C4H12N2)[Ni2(C6H5O7)2(H2O)4] | F(000) = 680 |
Mr = 655.80 | Dx = 1.869 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2yn | Cell parameters from 9899 reflections |
a = 13.342 (3) Å | θ = 3.0–27.6° |
b = 6.7054 (13) Å | µ = 1.71 mm−1 |
c = 13.613 (3) Å | T = 298 K |
β = 106.93 (3)° | Block, green |
V = 1165.1 (5) Å3 | 0.20 × 0.18 × 0.15 mm |
Z = 2 | |
Data collection top
Rigaku R-AXIS RAPID diffractometer | 2462 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.046 |
Graphite monochromator | θmax = 27.5°, θmin = 3.1° |
Oscillation scans | h = −17→17 |
10894 measured reflections | k = −8→8 |
2677 independent reflections | l = −17→17 |
Refinement top
Refinement on F2 | Primary atom site location: patt |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.029 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.073 | H-atom parameters constrained |
S = 1.08 | w = 1/[σ2(Fo2) + (0.033P)2 + 0.5209P] where P = (Fo2 + 2Fc2)/3 |
2677 reflections | (Δ/σ)max < 0.001 |
172 parameters | Δρmax = 0.36 e Å−3 |
0 restraints | Δρmin = −0.58 e Å−3 |
Crystal data top
(C4H12N2)[Ni2(C6H5O7)2(H2O)4] | V = 1165.1 (5) Å3 |
Mr = 655.80 | Z = 2 |
Monoclinic, P21/n | Mo Kα radiation |
a = 13.342 (3) Å | µ = 1.71 mm−1 |
b = 6.7054 (13) Å | T = 298 K |
c = 13.613 (3) Å | 0.20 × 0.18 × 0.15 mm |
β = 106.93 (3)° | |
Data collection top
Rigaku R-AXIS RAPID diffractometer | 2462 reflections with I > 2σ(I) |
10894 measured reflections | Rint = 0.046 |
2677 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.029 | 0 restraints |
wR(F2) = 0.073 | H-atom parameters constrained |
S = 1.08 | Δρmax = 0.36 e Å−3 |
2677 reflections | Δρmin = −0.58 e Å−3 |
172 parameters | |
Special details top
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell e.s.d.'s are taken
into account individually in the estimation of e.s.d.'s in distances, angles
and torsion angles; correlations between e.s.d.'s in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.
planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F2, conventional
R-factors R are based on F, with F set to zero for
negative F2. The threshold expression of F2 >
σ(F2) is used only for calculating R-factors(gt) etc.
and is not relevant to the choice of reflections for refinement.
R-factors based on F2 are statistically about twice as large
as those based on F, and R- factors based on ALL data will be
even larger. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
Ni1 | 0.356449 (16) | 0.10184 (3) | 0.089492 (15) | 0.01502 (9) | |
O1 | 0.46895 (9) | 0.27879 (17) | 0.05413 (8) | 0.0168 (2) | |
H1 | 0.4979 | 0.2121 | 0.0167 | 0.020* | |
O1W | 0.21588 (10) | 0.1791 (2) | −0.01518 (10) | 0.0267 (3) | |
H1WA | 0.2010 | 0.0965 | −0.0647 | 0.032* | |
H1WB | 0.2207 | 0.2945 | −0.0392 | 0.032* | |
O2 | 0.48547 (10) | 0.00460 (18) | 0.19579 (9) | 0.0210 (3) | |
O2W | 0.26483 (10) | −0.07943 (18) | 0.14890 (10) | 0.0215 (3) | |
H2WB | 0.2269 | −0.0091 | 0.1760 | 0.026* | |
H2WA | 0.3034 | −0.1542 | 0.1951 | 0.026* | |
O3 | 0.62422 (12) | 0.1371 (2) | 0.30710 (10) | 0.0308 (3) | |
O4 | 0.34262 (10) | 0.33467 (19) | 0.18366 (10) | 0.0225 (3) | |
O5 | 0.38093 (11) | 0.60449 (18) | 0.27900 (11) | 0.0247 (3) | |
O6 | 0.62307 (10) | 0.11233 (19) | 0.00955 (10) | 0.0239 (3) | |
O7 | 0.78853 (11) | 0.1785 (2) | 0.09831 (11) | 0.0329 (3) | |
N1 | 0.00054 (13) | 0.3898 (2) | 0.09153 (11) | 0.0207 (3) | |
H1A | 0.0278 | 0.2936 | 0.1377 | 0.025* | |
H1B | −0.0523 | 0.4481 | 0.1095 | 0.025* | |
C1 | 0.54617 (13) | 0.3207 (2) | 0.15133 (12) | 0.0155 (3) | |
C2 | 0.55405 (13) | 0.1389 (2) | 0.22351 (12) | 0.0165 (3) | |
C3 | 0.50727 (14) | 0.5019 (2) | 0.19770 (13) | 0.0191 (3) | |
H3A | 0.5601 | 0.5372 | 0.2610 | 0.023* | |
H3B | 0.5008 | 0.6132 | 0.1507 | 0.023* | |
C4 | 0.40335 (14) | 0.4756 (2) | 0.22078 (13) | 0.0172 (3) | |
C5 | 0.65218 (14) | 0.3690 (3) | 0.13446 (14) | 0.0200 (3) | |
H5A | 0.6457 | 0.4922 | 0.0958 | 0.024* | |
H5B | 0.7032 | 0.3910 | 0.2007 | 0.024* | |
C6 | 0.69299 (14) | 0.2077 (3) | 0.07807 (12) | 0.0193 (3) | |
C7 | 0.08284 (14) | 0.5411 (3) | 0.09266 (13) | 0.0226 (4) | |
H7A | 0.1424 | 0.4760 | 0.0792 | 0.027* | |
H7B | 0.1065 | 0.6024 | 0.1600 | 0.027* | |
C8 | 0.04050 (15) | 0.7003 (3) | 0.01229 (13) | 0.0227 (4) | |
H8A | −0.0152 | 0.7730 | 0.0291 | 0.027* | |
H8B | 0.0959 | 0.7941 | 0.0123 | 0.027* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
Ni1 | 0.01363 (13) | 0.01736 (13) | 0.01496 (13) | −0.00038 (8) | 0.00552 (9) | −0.00189 (8) |
O1 | 0.0153 (5) | 0.0214 (6) | 0.0144 (5) | 0.0000 (5) | 0.0054 (4) | −0.0003 (5) |
O1W | 0.0247 (7) | 0.0296 (7) | 0.0237 (6) | 0.0077 (6) | 0.0039 (5) | 0.0015 (6) |
O2 | 0.0214 (6) | 0.0190 (6) | 0.0206 (6) | −0.0021 (5) | 0.0029 (5) | 0.0026 (5) |
O2W | 0.0207 (6) | 0.0231 (6) | 0.0238 (6) | 0.0001 (5) | 0.0112 (5) | 0.0014 (5) |
O3 | 0.0296 (7) | 0.0320 (7) | 0.0225 (6) | −0.0021 (6) | −0.0056 (6) | 0.0017 (6) |
O4 | 0.0202 (6) | 0.0222 (6) | 0.0286 (6) | −0.0044 (5) | 0.0123 (5) | −0.0088 (6) |
O5 | 0.0268 (7) | 0.0219 (6) | 0.0306 (7) | −0.0042 (5) | 0.0168 (6) | −0.0100 (5) |
O6 | 0.0162 (6) | 0.0321 (7) | 0.0240 (6) | −0.0009 (5) | 0.0067 (5) | −0.0123 (5) |
O7 | 0.0160 (6) | 0.0441 (8) | 0.0376 (7) | 0.0008 (6) | 0.0062 (6) | −0.0163 (7) |
N1 | 0.0228 (8) | 0.0223 (7) | 0.0181 (7) | 0.0029 (6) | 0.0078 (6) | 0.0051 (6) |
C1 | 0.0142 (7) | 0.0168 (7) | 0.0163 (7) | −0.0022 (6) | 0.0056 (6) | −0.0041 (6) |
C2 | 0.0158 (8) | 0.0181 (7) | 0.0158 (7) | 0.0020 (7) | 0.0051 (6) | −0.0027 (6) |
C3 | 0.0194 (8) | 0.0158 (7) | 0.0244 (8) | −0.0016 (7) | 0.0101 (7) | −0.0048 (7) |
C4 | 0.0188 (8) | 0.0158 (7) | 0.0183 (7) | 0.0021 (7) | 0.0075 (6) | 0.0011 (6) |
C5 | 0.0181 (8) | 0.0207 (8) | 0.0232 (8) | −0.0043 (7) | 0.0092 (7) | −0.0058 (7) |
C6 | 0.0175 (8) | 0.0237 (8) | 0.0185 (7) | −0.0018 (7) | 0.0080 (7) | −0.0019 (7) |
C7 | 0.0211 (8) | 0.0272 (9) | 0.0181 (8) | −0.0022 (8) | 0.0032 (7) | −0.0014 (7) |
C8 | 0.0258 (9) | 0.0197 (8) | 0.0235 (8) | −0.0034 (7) | 0.0085 (7) | −0.0018 (7) |
Geometric parameters (Å, º) top
Ni1—O2 | 2.0078 (14) | N1—C8ii | 1.487 (2) |
Ni1—O6i | 2.0419 (13) | N1—C7 | 1.492 (2) |
Ni1—O2W | 2.0498 (13) | N1—H1A | 0.9000 |
Ni1—O4 | 2.0622 (12) | N1—H1B | 0.9000 |
Ni1—O1W | 2.0638 (15) | C1—C3 | 1.528 (2) |
Ni1—O1 | 2.0769 (12) | C1—C5 | 1.532 (2) |
O1—C1 | 1.448 (2) | C1—C2 | 1.550 (2) |
O1—H1 | 0.8499 | C3—C4 | 1.519 (2) |
O1W—H1WA | 0.8501 | C3—H3A | 0.9700 |
O1W—H1WB | 0.8500 | C3—H3B | 0.9700 |
O2—C2 | 1.261 (2) | C5—C6 | 1.516 (2) |
O2W—H2WB | 0.8501 | C5—H5A | 0.9700 |
O2W—H2WA | 0.8499 | C5—H5B | 0.9700 |
O3—C2 | 1.245 (2) | C7—C8 | 1.515 (3) |
O4—C4 | 1.252 (2) | C7—H7A | 0.9700 |
O5—C4 | 1.266 (2) | C7—H7B | 0.9700 |
O6—C6 | 1.281 (2) | C8—N1ii | 1.487 (2) |
O6—Ni1i | 2.0419 (12) | C8—H8A | 0.9700 |
O7—C6 | 1.239 (2) | C8—H8B | 0.9700 |
| | | |
O2—Ni1—O6i | 89.77 (6) | O1—C1—C2 | 108.98 (13) |
O2—Ni1—O2W | 90.44 (5) | C3—C1—C2 | 109.40 (13) |
O6i—Ni1—O2W | 93.06 (6) | C5—C1—C2 | 111.48 (14) |
O2—Ni1—O4 | 90.63 (5) | O3—C2—O2 | 123.64 (16) |
O6i—Ni1—O4 | 175.09 (5) | O3—C2—C1 | 118.72 (15) |
O2W—Ni1—O4 | 91.84 (5) | O2—C2—C1 | 117.56 (14) |
O2—Ni1—O1W | 174.29 (5) | C4—C3—C1 | 115.69 (14) |
O6i—Ni1—O1W | 89.43 (6) | C4—C3—H3A | 108.4 |
O2W—Ni1—O1W | 83.96 (6) | C1—C3—H3A | 108.4 |
O4—Ni1—O1W | 90.65 (6) | C4—C3—H3B | 108.4 |
O2—Ni1—O1 | 80.03 (5) | C1—C3—H3B | 108.4 |
O6i—Ni1—O1 | 90.23 (5) | H3A—C3—H3B | 107.4 |
O2W—Ni1—O1 | 169.91 (5) | O4—C4—O5 | 121.66 (16) |
O4—Ni1—O1 | 85.02 (5) | O4—C4—C3 | 121.80 (15) |
O1W—Ni1—O1 | 105.63 (5) | O5—C4—C3 | 116.54 (15) |
C1—O1—Ni1 | 105.60 (9) | C6—C5—C1 | 114.11 (14) |
C1—O1—H1 | 108.9 | C6—C5—H5A | 108.7 |
Ni1—O1—H1 | 108.8 | C1—C5—H5A | 108.7 |
Ni1—O1W—H1WA | 109.8 | C6—C5—H5B | 108.7 |
Ni1—O1W—H1WB | 109.8 | C1—C5—H5B | 108.7 |
H1WA—O1W—H1WB | 108.3 | H5A—C5—H5B | 107.6 |
C2—O2—Ni1 | 112.28 (11) | O7—C6—O6 | 124.52 (16) |
Ni1—O2W—H2WB | 109.9 | O7—C6—C5 | 119.87 (16) |
Ni1—O2W—H2WA | 109.8 | O6—C6—C5 | 115.59 (15) |
H2WB—O2W—H2WA | 108.4 | N1—C7—C8 | 110.69 (15) |
C4—O4—Ni1 | 131.14 (11) | N1—C7—H7A | 109.5 |
C6—O6—Ni1i | 128.50 (12) | C8—C7—H7A | 109.5 |
C8ii—N1—C7 | 110.63 (14) | N1—C7—H7B | 109.5 |
C8ii—N1—H1A | 109.5 | C8—C7—H7B | 109.5 |
C7—N1—H1A | 109.5 | H7A—C7—H7B | 108.1 |
C8ii—N1—H1B | 109.5 | N1ii—C8—C7 | 110.87 (14) |
C7—N1—H1B | 109.5 | N1ii—C8—H8A | 109.5 |
H1A—N1—H1B | 108.1 | C7—C8—H8A | 109.5 |
O1—C1—C3 | 107.15 (13) | N1ii—C8—H8B | 109.5 |
O1—C1—C5 | 110.25 (13) | C7—C8—H8B | 109.5 |
C3—C1—C5 | 109.47 (14) | H8A—C8—H8B | 108.1 |
Symmetry codes: (i) −x+1, −y, −z; (ii) −x, −y+1, −z. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O6 | 0.85 | 1.83 | 2.5632 (18) | 144 |
O1W—H1WA···O7i | 0.85 | 1.91 | 2.645 (2) | 143 |
O2W—H2WB···O5iii | 0.85 | 1.88 | 2.7163 (19) | 167 |
O2W—H2WA···O5iv | 0.85 | 2.08 | 2.903 (2) | 165 |
N1—H1A···O5iii | 0.90 | 1.89 | 2.765 (2) | 163 |
N1—H1B···O3v | 0.90 | 2.11 | 2.960 (2) | 157 |
Symmetry codes: (i) −x+1, −y, −z; (iii) −x+1/2, y−1/2, −z+1/2; (iv) x, y−1, z; (v) −x+1/2, y+1/2, −z+1/2. |
Experimental details
Crystal data |
Chemical formula | (C4H12N2)[Ni2(C6H5O7)2(H2O)4] |
Mr | 655.80 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 298 |
a, b, c (Å) | 13.342 (3), 6.7054 (13), 13.613 (3) |
β (°) | 106.93 (3) |
V (Å3) | 1165.1 (5) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 1.71 |
Crystal size (mm) | 0.20 × 0.18 × 0.15 |
|
Data collection |
Diffractometer | Rigaku R-AXIS RAPID |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 10894, 2677, 2462 |
Rint | 0.046 |
(sin θ/λ)max (Å−1) | 0.649 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.029, 0.073, 1.08 |
No. of reflections | 2677 |
No. of parameters | 172 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.36, −0.58 |
Selected geometric parameters (Å, º) topNi1—O2 | 2.0078 (14) | Ni1—O4 | 2.0622 (12) |
Ni1—O6i | 2.0419 (13) | Ni1—O1W | 2.0638 (15) |
Ni1—O2W | 2.0498 (13) | Ni1—O1 | 2.0769 (12) |
| | | |
O2—Ni1—O6i | 89.77 (6) | O2W—Ni1—O1W | 83.96 (6) |
O2—Ni1—O2W | 90.44 (5) | O4—Ni1—O1W | 90.65 (6) |
O6i—Ni1—O2W | 93.06 (6) | O2—Ni1—O1 | 80.03 (5) |
O2—Ni1—O4 | 90.63 (5) | O6i—Ni1—O1 | 90.23 (5) |
O6i—Ni1—O4 | 175.09 (5) | O2W—Ni1—O1 | 169.91 (5) |
O2W—Ni1—O4 | 91.84 (5) | O4—Ni1—O1 | 85.02 (5) |
O2—Ni1—O1W | 174.29 (5) | O1W—Ni1—O1 | 105.63 (5) |
O6i—Ni1—O1W | 89.43 (6) | | |
Symmetry code: (i) −x+1, −y, −z. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O6 | 0.85 | 1.83 | 2.5632 (18) | 143.7 |
O1W—H1WA···O7i | 0.85 | 1.91 | 2.645 (2) | 143.2 |
O2W—H2WB···O5ii | 0.85 | 1.88 | 2.7163 (19) | 166.9 |
O2W—H2WA···O5iii | 0.85 | 2.08 | 2.903 (2) | 164.6 |
N1—H1A···O5ii | 0.90 | 1.89 | 2.765 (2) | 163.4 |
N1—H1B···O3iv | 0.90 | 2.11 | 2.960 (2) | 157.3 |
Symmetry codes: (i) −x+1, −y, −z; (ii) −x+1/2, y−1/2, −z+1/2; (iii) x, y−1, z; (iv) −x+1/2, y+1/2, −z+1/2. |
Subscribe to Acta Crystallographica Section E: Crystallographic Communications
The full text of this article is available to subscribers to the journal.
If you have already registered and are using a computer listed in your registration details, please email
support@iucr.org for assistance.
Up to now, hundreds of metal citrate complexes with diverse architectures have been synthesized and well documented in the literature (Kaliva et al., 2004; Kefalas et al., 2005; Wang et al., 2005; Xiang et al., 2005; Zhang et al., 2006). Some complexes contain centrosymmetric dimers with 1-D polymeric chain or 2-D layer structure (Zhou et al., 2005; Baggio & Perec, 2004), some are similar to the title complex (Baker et al., 1983; Kotsakis et al., 2003), most of them have monovalent counter ions. In this paper, the complex we report has a divalent organic piperazinium cation. Its structure is shown in Fig. 1. Each citrate ligand is triply deprotonated, and chelates to the Ni atom through the α-hydroxyl, α-carboxyl and one β-carboxyl oxygen atom. The other β-carboxyl oxygen atom spans over to the second Ni atom of the dimer. The distorted octahedral coordination sphere of each nickel atom is completed by the oxygen atoms of two water molecules. Selected geometric parameters of the complex are given in Table 1. The piperazinium cations occupy the space between the nickel-citrate dimers. The anions and the cations are connected by strong N—H···O hydrogen bonds. There are intramolecular hydrogen bonds between the hydroxyl groups and the carboxyl groups. Hydrogen bonding interactions are also observed between the coordinated water molecules and the carboxyl groups of neighbouring anions, forming a three-dimensional network (Table 2, Fig. 2).