Chemical context   {#sec1}
==================

Platinum drugs are some of the most important and clinically applied anti-cancer agents. Di­phospho­nic acids are therapeutic agents for treating osteoporosis and metastatic bone diseases. Therefore new complexes designed with a combin­ation of platinum group metals with di­phospho­nic acid (or derivatives thereof) as bone-targeting groups can improve the chemotherapeutic efficacy in the treatment of bone cancer and can reduce adverse effects. Methyl­enedi­phospho­nic acid (medronic acid, MDP, H~4~ *L*) is the smallest bis­phospho­nate, which accumulates on the sites of osteoid mineralization and can be used in combination with platinum metals to treat cancer and metastatic bone diseases. Platinum--bis­phospho­nate complexes, including bis­{ethyl­enedi­amine)­platinum(II)}medronate, as novel Pt-prodrugs in the local treatment of bone tumors have been reported (Wani *et al.*, 2016[@bb14]; Iafisco *et al.*, 2009[@bb6]; Palazzo *et al.*, 2007[@bb10]; Iafisco & Margiotta, 2012[@bb5]; Margiotta *et al.*, 2009[@bb9]). The preparation and structure determination of \[Pt~2~(cis-di­amino­hexa­ne)~2~(methyl­enedi­phospho­n­ate)\] was reported by Bau *et al.* (1988[@bb1]).

In this work, we synthesized a new platinum metal complex, *viz*. \[Pd~2~(C~2~H~8~N~2~)~2~(CH~2~O~6~P~2~)\]·4H~2~O or \[Pd~2~(en)~2~(MDP)\]·4H~2~O, and report here its mol­ecular and crystal structures.

Structural commentary   {#sec2}
=======================

The binuclear \[Pd~2~(en)~2~(MDP)\] complex mol­ecule is uncharged and exhibits point group symmetry 2, with the twofold rotation axis passing through the central C atom of the MDP ligand (Fig. 1[▸](#fig1){ref-type="fig"}). The Pd^II^ atom has a square-planar environment defined by two nitro­gen atoms (N1 and N2) of the chelating en ligand and two oxygen atoms (O2 and O3) of the bis-bidentately coordinating MDP ligand that bridges two symmetry-related Pd^II^ atoms into the binuclear complex. The deviation of the Pd1 site from the least-squares plane involving the ligand atoms N1, N2, O2 and O3 is 0.06 Å. The Pd---N and Pd---O bond lengths are in typical ranges whereby the Pd---N bonds are about 0.02 Å shorter than the Pd---O bonds (Table 1[▸](#table1){ref-type="table"}). The O---Pd---O, N---Pd---N and O---Pd---N bond angles vary within 84.5 (2)°--93.3 (2)° (Table 1[▸](#table1){ref-type="table"}) and indicate a slight distortion from a square-planar coordination. In general, the structural features of the \[Pd~2~(en)~2~(MDP)\] complex are similar to those of the related complex \[Pd~2~(en)~2~EDP\] where EDP is 1-hy­droxy­ethane 1,1-di­phospho­nate or etidronate (Kozachkova *et al.*, 2018[@bb7]).

The Pd1---O1---P1---C3---P1^i^---O1^i^ \[symmetry code: (i)  − *x*,  − *y*, *z*\] six-membered metallacycle adopts a chair conformation; the puckering parameters (Cremer & Pople, 1975[@bb2]; Zefirov *et al.*, 1990[@bb15]) are *S* = 1.19, *ψ* = 16.52°, *θ* = 3.02°. The Pd1 and C3 atoms deviate by −0.95 and 0.82 Å, respectively, from the least-squares plane of the remaining atoms of this metallacycle with an s.u. of 0.01 Å. The Pd1---N1---C2---C1---N2 five-membered metallacycle involving the en ligand adopts an envelope conformation. The N1 atom deviates by 0.27 Å from the mean plane of the remaining atoms (the s.u. of this plane is 0.005 Å).

Supra­molecular features   {#sec3}
==========================

In the crystal, \[Pd~2~(en)~2~(MDP)\] mol­ecules are linked via ~(en)~N1---H1*A*⋯O1^ii^ ~(MDP)~ hydrogen bonds (Fig. 2[▸](#fig2){ref-type="fig"}, Table 2[▸](#table2){ref-type="table"}), forming chains parallel to \[010\]. Neighboring chains are connected to each other through ~(en)~N2---H2*A*⋯O3^iii^ ~(MDP)~ and ~(en)~N1---H1*B*⋯O3^iv^ ~(MDP)~ hydrogen bonds, forming layers parallel to (001). The disordered water mol­ecules O4 and O5 are located between these layers and are connected to each other by O---H⋯O hydrogen bonds (Table 2[▸](#table2){ref-type="table"}), forming a sandwich-type structure. The structural disorder of the water mol­ecules is probably caused by different possibilities of possible positions favourable for hydrogen bonding with neighbouring mol­ecules. Alternating layers containing \[Pd~2~(en)~2~(MDP)\] complexes and water mol­ecules are stacked along \[001\] and are linked to each other by a series of O---H⋯O hydrogen bonds (O5*A*---H5*AB*⋯O3 and O4*B*---H4*BB*⋯O3; Table 2[▸](#table2){ref-type="table"}).

Database survey   {#sec4}
=================

A search of the Cambridge Structural Database (CSD, Version 5.39, update November 2017; Groom *et al.*, 2016[@bb4]) for complexes containing the Pd(en) moiety yielded 226 hits with a mean Pd---N bond lengths of 2.028 Å. A search for Pd complexes with MPD as a ligand revealed only one entry (Kutsenko *et al.*, 2014[@bb8]) with Pd---O bond lengths of 1.999 and 2.004 Å.

Synthesis and crystallization   {#sec5}
===============================

\[Pd~2~(C~2~H~8~N~2~)~2~(CH~2~O~6~P~2~)\]·4H~2~O was prepared as previously reported in the literature (Kozachkova *et al.*, 2018[@bb7]). A solution of AgNO~3~ (0.4 mmol, 0.0679 g) in water (3 ml) was added to a suspension of \[Pd(en)Cl~2~\] (0.2 mmol, 0.0474 g) in water (6 ml) under constant stirring and heating at 333 K for 30 min. The resulting suspension was refrigerated to facilitate the precipitation of AgCl. The solid material was removed by suction filtration. MDP (0.1 mmol, 0.0176 g) was added to the filtrate, and the pH was adjusted to 6 by addition of KOH (0.1 mol l^−1^). The obtained slightly yellow solution was heated at 333 K for 20 min and then left to evaporate at room temperature. Yellow rectangular crystals of the title compound suitable for crystallographic studies were grown by slow evaporation of an aqueous solution at room temperature.

Compound \[Pd~2~(en)~2~(MDP)\]·4H~2~O: yield 86%. Analysis found: C, 11.5; H, 3.6; N, 10.9; P, 11.8; Pd, 40.8. Calculated for C~5~H~26~N~4~O~6~P~2~Pd~2~: C, 11.9; H, 3.9; N, 11.1; P, 12.3; Pd, 41.9%. The ^31^P-{H} NMR spectrum of an aqueous solutions of the synthesized compound exhibited a singlet with δ^P^ 35.0 ppm.

Refinement   {#sec6}
============

Crystal data, data collection and structure refinement details are summarized in Table 3[▸](#table3){ref-type="table"}. Hydrogen atoms of the metal complex were located from difference-Fourier syntheses. The H atom of the central CH~2~ bridge of the MDP ligand was refined freely, and the methyl­ene and NH~2~ hydrogen atoms of the en ligand were treated in the riding-model approximation with C---H = 0.97 Å (N---H = 0.89 Å) and *U* ~iso~(H) = 1.2*U* ~eq~(C,N). The two lattice water mol­ecules (O4 and O5) are each disordered over two positions and were refined with occupancies 0.5:0.5 each. Their hydrogen atoms were calculated, taking into account the direction of expected hydrogen bonds. The positions of these hydrogen atoms were fixed at the last steps of refinement with O---H = 0.85 Å, and with *U* ~iso~(H) = 1.5*U* ~eq~(O).

Supplementary Material
======================

Crystal structure: contains datablock(s) I. DOI: [10.1107/S2056989018016419/wm5473sup1.cif](https://doi.org/10.1107/S2056989018016419/wm5473sup1.cif)

Structure factors: contains datablock(s) I. DOI: [10.1107/S2056989018016419/wm5473Isup2.hkl](https://doi.org/10.1107/S2056989018016419/wm5473Isup2.hkl)

CCDC reference: [1879750](http://scripts.iucr.org/cgi-bin/cr.cgi?rm=csd&csdid=1879750)

Additional supporting information: [crystallographic information](http://scripts.iucr.org/cgi-bin/sendsupfiles?wm5473&file=wm5473sup0.html&mime=text/html); [3D view](http://scripts.iucr.org/cgi-bin/sendcif?wm5473sup1&Qmime=cif); [checkCIF report](http://scripts.iucr.org/cgi-bin/paper?wm5473&checkcif=yes)

Crystal data {#d1e133}
============

  -------------------------------------------------- ---------------------------------------
  \[Pd~2~(C~2~H~8~N~2~)~2~(CH~2~O~6~P~2~)\]·4H~2~O   *D*~x~ = 2.140 Mg m^−3^
  *M~r~* = 577.04                                    Mo *K*α radiation, λ = 0.71073 Å
  Orthorhombic, *Fddd*                               Cell parameters from 2899 reflections
  *a* = 11.8871 (6) Å                                θ = 3.2--30.1°
  *b* = 12.5405 (6) Å                                µ = 2.24 mm^−1^
  *c* = 48.052 (2) Å                                 *T* = 294 K
  *V* = 7163.2 (6) Å^3^                              Block, colourless
  *Z* = 16                                           0.6 × 0.4 × 0.2 mm
  *F*(000) = 4576                                    
  -------------------------------------------------- ---------------------------------------

Data collection {#d1e273}
===============

  --------------------------------------------------------------------------- --------------------------------------
  Rigaku Oxford Diffraction Xcalibur, Sapphire3 diffractometer                2065 independent reflections
  Radiation source: fine-focus sealed X-ray tube, Enhance (Mo) X-ray Source   1746 reflections with *I* \> 2σ(*I*)
  Graphite monochromator                                                      *R*~int~ = 0.075
  Detector resolution: 16.1827 pixels mm^-1^                                  θ~max~ = 27.5°, θ~min~ = 3.2°
  ω scans                                                                     *h* = −15→8
  Absorption correction: multi-scan (*CrysAlisPro*; Rigaku OD, 2018)          *k* = −16→16
  *T*~min~ = 0.418, *T*~max~ = 1.000                                          *l* = −58→62
  14597 measured reflections                                                  
  --------------------------------------------------------------------------- --------------------------------------

Refinement {#d1e393}
==========

  ------------------------------------- ---------------------------------------------------------------------------------------------------
  Refinement on *F*^2^                  0 restraints
  Least-squares matrix: full            Hydrogen site location: mixed
  *R*\[*F*^2^ \> 2σ(*F*^2^)\] = 0.063   H-atom parameters constrained
  *wR*(*F*^2^) = 0.175                  *w* = 1/\[σ^2^(*F*~o~^2^) + (0.0908*P*)^2^ + 141.6934*P*\] where *P* = (*F*~o~^2^ + 2*F*~c~^2^)/3
  *S* = 1.14                            (Δ/σ)~max~ = 0.002
  2065 reflections                      Δρ~max~ = 1.69 e Å^−3^
  126 parameters                        Δρ~min~ = −2.06 e Å^−3^
  ------------------------------------- ---------------------------------------------------------------------------------------------------

Special details {#d1e544}
===============

  -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
  -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å^2^) {#d1e563}
==================================================================================================

  ------ -------------- -------------- -------------- -------------------- ------------
         *x*            *y*            *z*            *U*~iso~\*/*U*~eq~   Occ. (\<1)
  Pd1    0.37981 (4)    0.24678 (4)    0.61611 (2)    0.0284 (2)           
  P1     0.24784 (12)   0.37098 (12)   0.66205 (3)    0.0273 (4)           
  O1     0.2537 (4)     0.2699 (4)     0.64405 (11)   0.0346 (10)          
  O2     0.2468 (4)     0.4739 (4)     0.64479 (10)   0.0322 (10)          
  O3     0.1464 (4)     0.3667 (4)     0.68076 (9)    0.0362 (10)          
  N2     0.4951 (5)     0.2116 (5)     0.58694 (13)   0.0387 (13)          
  H2A    0.542935       0.265756       0.585250       0.046\*              
  H2B    0.533949       0.154405       0.592230       0.046\*              
  N1     0.2672 (5)     0.2073 (5)     0.58635 (12)   0.0355 (12)          
  H1A    0.235915       0.144735       0.590388       0.043\*              
  H1B    0.212909       0.256180       0.585660       0.043\*              
  C3     0.375000       0.375000       0.68243 (19)   0.0299 (17)          
  H3     0.373 (6)      0.437 (6)      0.6938 (17)    0.036\*              
  C1     0.3239 (8)     0.2011 (11)    0.55937 (18)   0.073 (3)            
  H1C    0.293176       0.140753       0.549266       0.087\*              
  H1D    0.305487       0.264816       0.548883       0.087\*              
  C2     0.4418 (8)     0.1906 (13)    0.5599 (2)     0.089 (4)            
  H2C    0.473610       0.239097       0.546319       0.107\*              
  H2D    0.461070       0.118672       0.554208       0.107\*              
  O4A    0.1779 (11)    0.1374 (12)    0.7512 (3)     0.072 (4)            0.5
  H4AA   0.162933       0.203516       0.754198       0.108\*              0.5
  H4AB   0.115953       0.105016       0.747549       0.108\*              0.5
  O5B    0.1617 (16)    0.5219 (16)    0.7487 (5)     0.108 (7)            0.5
  H5BA   0.226607       0.542097       0.753922       0.163\*              0.5
  H5BB   0.118927       0.574297       0.752362       0.163\*              0.5
  O5A    0.1535 (13)    0.3512 (16)    0.7363 (3)     0.064 (4)            0.5
  H5AA   0.084422       0.344140       0.740362       0.096\*              0.5
  H5AB   0.154092       0.358020       0.718632       0.096\*              0.5
  O4B    0.1903 (15)    0.2992 (16)    0.7344 (4)     0.069 (5)            0.5
  H4BA   0.228698       0.242206       0.733009       0.104\*              0.5
  H4BB   0.164928       0.316206       0.718439       0.104\*              0.5
  ------ -------------- -------------- -------------- -------------------- ------------

Atomic displacement parameters (Å^2^) {#d1e986}
=====================================

  ----- ------------ ------------ ------------ -------------- --------------- ---------------
        *U*^11^      *U*^22^      *U*^33^      *U*^12^        *U*^13^         *U*^23^
  Pd1   0.0273 (3)   0.0295 (4)   0.0285 (4)   0.00014 (16)   −0.00047 (18)   −0.00197 (17)
  P1    0.0229 (7)   0.0320 (8)   0.0270 (8)   −0.0004 (5)    0.0012 (6)      0.0012 (6)
  O1    0.028 (2)    0.041 (3)    0.035 (3)    −0.0021 (18)   0.0028 (19)     −0.005 (2)
  O2    0.028 (2)    0.035 (2)    0.034 (2)    0.0019 (17)    0.0010 (18)     0.0029 (19)
  O3    0.026 (2)    0.051 (3)    0.031 (2)    0.0004 (18)    0.0045 (18)     −0.001 (2)
  N2    0.032 (3)    0.047 (3)    0.036 (3)    0.000 (2)      0.002 (2)       0.000 (3)
  N1    0.031 (3)    0.040 (3)    0.035 (3)    −0.003 (2)     −0.006 (2)      −0.005 (2)
  C3    0.028 (4)    0.036 (5)    0.025 (4)    −0.001 (3)     0.000           0.000
  C1    0.061 (6)    0.126 (10)   0.031 (4)    0.010 (6)      −0.010 (4)      −0.013 (5)
  C2    0.047 (5)    0.176 (13)   0.045 (5)    −0.020 (7)     0.009 (4)       −0.028 (7)
  O4A   0.058 (7)    0.081 (10)   0.077 (10)   0.006 (7)      −0.006 (7)      −0.007 (8)
  O5B   0.074 (11)   0.117 (15)   0.134 (17)   −0.013 (10)    0.010 (11)      −0.061 (13)
  O5A   0.046 (8)    0.114 (16)   0.032 (6)    0.000 (8)      0.002 (5)       −0.002 (8)
  O4B   0.064 (11)   0.097 (14)   0.047 (8)    0.001 (9)      0.009 (7)       0.002 (9)
  ----- ------------ ------------ ------------ -------------- --------------- ---------------

Geometric parameters (Å, º) {#d1e1281}
===========================

  ------------------------ -------------- ------------------------ ------------
  Pd1---Pd1^i^             3.2180 (9)     C3---H3                  0.95 (8)
  Pd1---Pd1^ii^            3.1715 (9)     C3---H3^i^               0.95 (8)
  Pd1---O1                 2.033 (5)      C1---H1C                 0.9700
  Pd1---O2^i^              2.046 (5)      C1---H1D                 0.9700
  Pd1---N2                 2.009 (6)      C1---C2                  1.408 (14)
  Pd1---N1                 2.021 (5)      C2---H2C                 0.9700
  P1---O1                  1.536 (5)      C2---H2D                 0.9700
  P1---O2                  1.534 (5)      O4A---H4AA               0.8600
  P1---O3                  1.506 (4)      O4A---H4AB               0.8590
  P1---C3                  1.802 (5)      O4A---H4AB^iii^          0.56 (2)
  N2---H2A                 0.8900         O5B---H5BA               0.8503
  N2---H2B                 0.8900         O5B---H5BB               0.8504
  N2---C2                  1.468 (11)     O5A---H5AA               0.8495
  N1---H1A                 0.8900         O5A---H5AB               0.8515
  N1---H1B                 0.8900         O4B---H4BA               0.8502
  N1---C1                  1.464 (11)     O4B---H4BB               0.8500
                                                                   
  Pd1^ii^---Pd1---Pd1^i^   164.246 (17)   Pd1---N1---H1B           109.8
  O1---Pd1---Pd1^i^        80.27 (14)     H1A---N1---H1B           108.2
  O1---Pd1---Pd1^ii^       87.69 (14)     C1---N1---Pd1            109.6 (5)
  O1---Pd1---O2^i^         93.32 (19)     C1---N1---H1A            109.8
  O2^i^---Pd1---Pd1^i^     81.11 (13)     C1---N1---H1B            109.8
  O2^i^---Pd1---Pd1^ii^    89.54 (13)     P1---C3---P1^i^          114.2 (5)
  N2---Pd1---Pd1^i^        104.11 (19)    P1---C3---H3             108 (4)
  N2---Pd1---Pd1^ii^       88.64 (19)     P1---C3---H3^i^          108 (4)
  N2---Pd1---O1            174.2 (2)      P1^i^---C3---H3^i^       108 (4)
  N2---Pd1---O2^i^         91.2 (2)       P1^i^---C3---H3          108 (4)
  N2---Pd1---N1            84.5 (2)       H3---C3---H3^i^          110 (10)
  N1---Pd1---Pd1^ii^       87.42 (18)     N1---C1---H1C            108.2
  N1---Pd1---Pd1^i^        102.77 (18)    N1---C1---H1D            108.2
  N1---Pd1---O1            90.8 (2)       H1C---C1---H1D           107.3
  N1---Pd1---O2^i^         174.8 (2)      C2---C1---N1             116.5 (8)
  O1---P1---C3             106.9 (2)      C2---C1---H1C            108.2
  O2---P1---O1             113.0 (3)      C2---C1---H1D            108.2
  O2---P1---C3             106.1 (2)      N2---C2---H2C            108.4
  O3---P1---O1             110.1 (3)      N2---C2---H2D            108.4
  O3---P1---O2             110.3 (3)      C1---C2---N2             115.4 (8)
  O3---P1---C3             110.4 (3)      C1---C2---H2C            108.4
  P1---O1---Pd1            121.6 (3)      C1---C2---H2D            108.4
  P1---O2---Pd1^i^         120.7 (3)      H2C---C2---H2D           107.5
  Pd1---N2---H2A           109.4          H4AA---O4A---H4AB        108.2
  Pd1---N2---H2B           109.4          H4AA---O4A---H4AB^iii^   72.3
  H2A---N2---H2B           108.0          H4AB---O4A---H4AB^iii^   38.4
  C2---N2---Pd1            111.2 (5)      H5BA---O5B---H5BB        104.5
  C2---N2---H2A            109.4          H5AA---O5A---H5AB        104.4
  C2---N2---H2B            109.4          H4BA---O4B---H4BB        109.4
  Pd1---N1---H1A           109.8                                   
                                                                   
  Pd1---N2---C2---C1       1.3 (14)       O3---P1---O1---Pd1       −179.9 (3)
  Pd1---N1---C1---C2       18.4 (13)      O3---P1---O2---Pd1^i^    −177.9 (3)
  O1---P1---O2---Pd1^i^    −54.3 (4)      O3---P1---C3---P1^i^     179.4 (2)
  O1---P1---C3---P1^i^     59.7 (2)       N1---C1---C2---N2        −13.4 (17)
  O2---P1---O1---Pd1       56.3 (4)       C3---P1---O1---Pd1       −60.0 (4)
  O2---P1---C3---P1^i^     −61.1 (2)      C3---P1---O2---Pd1^i^    62.5 (4)
  ------------------------ -------------- ------------------------ ------------

Symmetry codes: (i) −*x*+3/4, −*y*+3/4, *z*; (ii) *x*, −*y*+1/4, −*z*+5/4; (iii) −*x*+1/4, −*y*+1/4, *z*.

Hydrogen-bond geometry (Å, º) {#d1e1910}
=============================

  ------------------------------ --------- --------- ------------ ---------------
  *D*---H···*A*                  *D*---H   H···*A*   *D*···*A*    *D*---H···*A*
  N2---H2*A*···O3^iv^            0.89      2.07      2.932 (8)    163
  N1---H1*A*···O1^ii^            0.89      2.21      3.006 (8)    149
  N1---H1*B*···O3^v^             0.89      2.06      2.906 (8)    159
  O4*A*---H4*AA*···O5*A*         0.86      2.05      2.79 (3)     145
  O5*B*---H5*BA*···O5*B*^vi^     0.85      1.56      2.17 (4)     127
  O5*B*---H5*BA*···O4*B*^vi^     0.85      2.29      2.97 (3)     136
  O5*B*---H5*BB*···O5*B*^vii^    0.85      1.97      2.73 (4)     148
  O5*B*---H5*BB*···O4*B*^viii^   0.85      2.24      2.78 (2)     121
  O5*A*---H5*AA*···O4*A*^viii^   0.85      1.98      2.786 (19)   158
  O5*A*---H5*AB*···O3            0.85      1.83      2.675 (16)   176
  O4*B*---H4*BB*···O3            0.85      1.93      2.761 (18)   165
  ------------------------------ --------- --------- ------------ ---------------

Symmetry codes: (ii) *x*, −*y*+1/4, −*z*+5/4; (iv) *x*+1/2, −*y*+3/4, −*z*+5/4; (v) −*x*+1/4, *y*, −*z*+5/4; (vi) −*x*+1/2, −*y*+1, −*z*+3/2; (vii) −*x*+1/4, −*y*+5/4, *z*; (viii) *x*−1/4, *y*+1/4, −*z*+3/2.

![Mol­ecular structure of the title complex and surrounding water mol­ecules with displacement ellipsoids drawn at the 50% probability level. \[Symmetry code: (i) −*x* + , −*y* + , *z*.\]](e-74-01838-fig1){#fig1}

![Crystal packing of the title compound in a view along \[010\] with hydrogen bonds shown as dashed lines.](e-74-01838-fig2){#fig2}

###### Selected geometric parameters (Å, °)

  ------------------ ------------ --------------- -----------
  Pd1---O1           2.033 (5)    Pd1---N2        2.009 (6)
  Pd1---O2^i^        2.046 (5)    Pd1---N1        2.021 (5)
                                                   
  O1---Pd1---O2^i^   93.32 (19)   N2---Pd1---N1   84.5 (2)
  N2---Pd1---O2^i^   91.2 (2)     N1---Pd1---O1   90.8 (2)
  ------------------ ------------ --------------- -----------

Symmetry code: (i) .

###### Hydrogen-bond geometry (Å, °)

  *D*---H⋯*A*                  *D*---H   H⋯*A*   *D*⋯*A*      *D*---H⋯*A*
  ---------------------------- --------- ------- ------------ -------------
  N2---H2*A*⋯O3^ii^            0.89      2.07    2.932 (8)    163
  N1---H1*A*⋯O1^iii^           0.89      2.21    3.006 (8)    149
  N1---H1*B*⋯O3^iv^            0.89      2.06    2.906 (8)    159
  O4*A*---H4*AA*⋯O5*A*         0.86      2.05    2.79 (3)     145
  O5*B*---H5*BA*⋯O5*B* ^v^     0.85      1.56    2.17 (4)     127
  O5*B*---H5*BA*⋯O4*B* ^v^     0.85      2.29    2.97 (3)     136
  O5*B*---H5*BB*⋯O5*B* ^vi^    0.85      1.97    2.73 (4)     148
  O5*B*---H5*BB*⋯O4*B* ^vii^   0.85      2.24    2.78 (2)     121
  O5*A*---H5*AA*⋯O4*A* ^vii^   0.85      1.98    2.786 (19)   158
  O5*A*---H5*AB*⋯O3            0.85      1.83    2.675 (16)   176
  O4*B*---H4*BB*⋯O3            0.85      1.93    2.761 (18)   165

Symmetry codes: (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) .

###### Experimental details

  -------------------------------------------------------------------------- -----------------------------------------------------
  Crystal data                                                               
  Chemical formula                                                           \[Pd~2~(C~2~H~8~N~2~)~2~(CH~2~O~6~P~2~)\]·4H~2~O
  *M* ~r~                                                                    577.04
  Crystal system, space group                                                Orthorhombic, *F* *d* *d* *d*
  Temperature (K)                                                            294
  *a*, *b*, *c* (Å)                                                          11.8871 (6), 12.5405 (6), 48.052 (2)
  *V* (Å^3^)                                                                 7163.2 (6)
  *Z*                                                                        16
  Radiation type                                                             Mo *K*α
  μ (mm^−1^)                                                                 2.24
  Crystal size (mm)                                                          0.6 × 0.4 × 0.2
                                                                             
  Data collection                                                            
  Diffractometer                                                             Rigaku Oxford Diffraction Xcalibur, Sapphire3
  Absorption correction                                                      Multi-scan (*CrysAlis PRO*; Rigaku OD, 2018[@bb11])
  *T* ~min~, *T* ~max~                                                       0.418, 1.000
  No. of measured, independent and observed \[*I* \> 2σ(*I*)\] reflections   14597, 2065, 1746
  *R* ~int~                                                                  0.075
  (sin θ/λ)~max~ (Å^−1^)                                                     0.649
                                                                             
  Refinement                                                                 
  *R*\[*F* ^2^ \> 2σ(*F* ^2^)\], *wR*(*F* ^2^), *S*                          0.063, 0.175, 1.14
  No. of reflections                                                         2065
  No. of parameters                                                          126
  H-atom treatment                                                           H-atom parameters constrained
  Δρ~max~, Δρ~min~ (e Å^−3^)                                                 1.69, −2.06
  -------------------------------------------------------------------------- -----------------------------------------------------

Computer programs: *CrysAlis PRO* (Rigaku OD, 2018[@bb11]), *SHELXT* (Sheldrick, 2015*a* [@bb12]), *SHELXL* (Sheldrick, 2015*b* [@bb13]) and *OLEX2* (Dolomanov *et al.*, 2009[@bb3]).
