Redetermination of nickel(II) formate dihydrate

In comparison with the previous structure determination of poly[di­aquadi-μ-formato-nickel(II)], [Ni(HCOO)₂(H₂O)₂]_n, based on Weissenberg film data [Krogmann & Mattes (1963). Z. Kristallogr. 118, 291–302], the current redetermination from modern CCD data revealed the positions of the H atoms, thus making a detailed description of the hydrogen-bonding pattern possible. Both Ni²⁺ cations in the crystal structure are located on inversion centres and are octa­hedrally coordinated. One Ni²⁺ cation is bound to six O atoms of six formate anions whereas the other Ni²⁺ cation is bound to four O atoms of water mol­ecules and to two formate O atoms. In this way, the formate anions bridge the two types of Ni²⁺ cations into a three-dimensional framework. O—H⋯O hydrogen bonds of medium strength between water mol­ecules and formate O atoms consolidate the packing.

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Chemical scheme

Structure description

Recycling of tungsten carbide from WC–Ni hard metals or composites thereof can be achieved by debinding WC–Ni with formic acid to selectively dissolve nickel. Nickel formate then can either be crystallized as the dihydrate from the obtained solution, or formic acid can be regenerated through cation exchange with sulfuric acid. In the latter case, nickel can be precipitated as Ni(OH)₂ from the inter­mediate nickel sulfate solution by adding caustic soda (Weissensteiner, 2012). In the course of these studies it became apparent that a redetermination of the crystal structure of nickel formate dihydrate, Ni(HCOO)₂·2H₂O, (Krogmann & Mattes, 1963) was desirable in terms of higher precision and accuracy and for an unambiguous assignment of the hydrogen-bonding scheme. Although a profile refinement using the Rietveld method has been performed on this material, leading to precise room-temperature lattice parameters (Kellerman et al., 2016), improved structural data are still missing.

The crystal structure of Ni(HCOO)₂·2H₂O comprises two Ni²⁺ cations on inversion centres, one on Wyckoff position 2b (Ni1), one on 2a (Ni2), and two formate anions and two water mol­ecules in general positions. The Ni²⁺ cations are stacked in rows parallel to [101]. Both cations have a distorted octa­hedral coordination environment by oxygen atoms, but with different types of ligands. Ni1 is bound to six O atoms of six formate anions (O1–O3 and symmetry-related counterparts), whereas Ni2 is bound to four O atoms of two pairs of water mol­ecules (O5, O6 and symmetry-related counterparts) and two formate anions (O4 and its symmetry-related counterpart). Relevant bond lengths and a comparison with the previous determination are collated in Table 1. In general, bond lengths and angles are similar to related divalent first-row transition metal formates (Viertelhaus et al., 2005).

Table 1
Comparison of bond lengths (Å) in the current and the previous refinement of Ni(HCOO)₂·2H₂O^(a,b)

	current refinement	previous refinement^a
Ni1—O1^i	2.0302 (6)	2.026 (8)
Ni1—O2	2.0503 (6)	2.061 (8)
Ni1—O3	2.0942 (6)	2.097 (8)
Ni2—O5	2.0256 (7)	2.042 (8)
Ni2—O6	2.0663 (6)	2.059 (8)
Ni2—O4	2.1006 (7)	2.090 (8)
O1—C1	1.2593 (10)	1.256 (8)
O2—C1	1.2546 (10)	1.222 (8)
O3—C2^ii	1.2618 (10)	1.278 (8)
O4—C2	1.2607 (10)	1.247 (8)
Symmetry codes: (i) −x + 1, y − , −z + ; (ii) x, −y + , z + . Notes: (a) Krogmann & Mattes (1963); lattice parameters a = 8.60 (1), b = 7.06 (1), c = 9.21 (2) Å, β = 96.50 (10)° from single-crystal data at room temperature; (b) lattice parameters a = 8.5951 (1), b = 7.0688 (5), c = 9.2152 (2) Å, β = 97.41 (1)° from Rietveld profile refinement at room temperature (Kellerman et al., 2016).

Each of the two formate anions bridges two Ni²⁺ cations, thus creating a three-dimensional framework. O—H⋯O hydrogen bonds of medium strength and with nearly linear O—H⋯O angles between water mol­ecules as donor groups and each of the formate carboxyl­ate O atoms as acceptor groups help to consolidate this arrangement (Fig. 1, Table 2). In comparison with the previous determination, the H-atom positions are unambiguous and were clearly discernible from difference maps.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O6—H5⋯O3^i	0.787 (18)	1.985 (18)	2.7312 (9)	158.1 (16)
O5—H3⋯O2	0.89 (2)	1.87 (2)	2.7522 (9)	171.2 (19)
O5—H4⋯O4^i	0.832 (18)	1.898 (18)	2.7271 (10)	174.1 (17)
O6—H6⋯O1^ii	0.837 (18)	1.926 (18)	2.7610 (9)	175.5 (16)
Symmetry codes: (i) ; (ii) -x, -y+2, -z.

Figure 1 The crystal structure of Ni(HCOO)2·2H2O in a projection along [00]. Displacement ellipsoids are drawn at the 97% probability level. Ni atoms are green, C atoms grey, formate O atoms red, water O atoms yellow. H atoms are shown as white spheres of arbitrary radius; O—H⋯O hydrogen bonding is indicated by thin blue lines.

Synthesis and crystallization

Crystals of the title compound were harvested from a saturated aqueous solution of nickel formate (Königswarter & Ebell, Chemische Fabrik GmbH, Germany) that was stored in a closed glass bottle for several months.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. Starting coordinates for refinement were taken from the previous determination (Krogmann & Mattes, 1963).

Table 3
Experimental details

Crystal data
Chemical formula	[Ni(HCOO)2(H2O)2]
Mr	184.78
Crystal system, space group	Monoclinic, P21/c
Temperature (K)	100
a, b, c (Å)	8.5806 (4), 7.0202 (3), 9.2257 (4)
β (°)	97.551 (1)
V (Å^3)	550.91 (4)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	3.48
Crystal size (mm)	0.12 × 0.10 × 0.02
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2015)
Tmin, Tmax	0.667, 0.748
No. of measured, independent and observed [I > 2σ(I)] reflections	44072, 3433, 2591
Rint	0.036
(sin θ/λ)max (Å^−1)	0.907
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.021, 0.049, 1.04
No. of reflections	3433
No. of parameters	101
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å^−3)	0.52, −0.55
Computer programs: APEX3 and SAINT (Bruker, 2015), SHELXL2016 (Sheldrick, 2015), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010). Coordinates from previous determination.

Structural data

Computing details

Data collection: APEX3 (Bruker, 2015); cell refinement: SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to solve structure: coordinates from previous determination; program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Poly[diaquadi-µ-formato-nickel(II)]

Crystal data

[Ni(CHO2)2(H2O)2]	F(000) = 376
Mr = 184.78	Dx = 2.228 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 8.5806 (4) Å	Cell parameters from 9891 reflections
b = 7.0202 (3) Å	θ = 2.4–39.9°
c = 9.2257 (4) Å	µ = 3.48 mm^−1
β = 97.551 (1)°	T = 100 K
V = 550.91 (4) Å^3	Plate, green
Z = 4	0.12 × 0.10 × 0.02 mm

Data collection

Bruker APEXII CCD diffractometer	2591 reflections with I > 2σ(I)
ω– and φ–scans	Rint = 0.036
Absorption correction: multi-scan (SADABS; Bruker, 2015)	θmax = 40.1°, θmin = 2.4°
Tmin = 0.667, Tmax = 0.748	h = −15→15
44072 measured reflections	k = −12→12
3433 independent reflections	l = −16→16

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Hydrogen site location: mixed
R[F^2 > 2σ(F^2)] = 0.021	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.049	w = 1/[σ^2(Fo^2) + (0.0181P)^2 + 0.2503P] where P = (Fo^2 + 2Fc^2)/3
S = 1.04	(Δ/σ)max = 0.001
3433 reflections	Δρmax = 0.52 e Å^−3
101 parameters	Δρmin = −0.55 e Å^−3

Special details

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.

Refinement. H atoms bound to O atoms were located from a difference map and were refined freely.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ni1	0.500000	1.000000	0.500000	0.00423 (3)
Ni2	0.000000	1.000000	0.000000	0.00564 (3)
O1	0.40966 (7)	1.27481 (9)	0.09909 (7)	0.00780 (10)
O2	0.40368 (7)	1.10493 (9)	0.30080 (7)	0.00783 (10)
O3	0.29319 (7)	0.84112 (9)	0.49822 (7)	0.00847 (10)
O4	0.06317 (8)	0.72530 (10)	0.07671 (8)	0.01179 (11)
O5	0.08832 (8)	1.11264 (12)	0.19580 (8)	0.01638 (14)
O6	−0.21606 (8)	0.97254 (10)	0.07381 (7)	0.00910 (10)
C1	0.46712 (10)	1.22417 (12)	0.22586 (9)	0.00844 (12)
H1	0.563988	1.279365	0.267077	0.010*
C2	0.17660 (10)	0.61599 (12)	0.06198 (10)	0.00963 (13)
H2	0.174105	0.491537	0.102054	0.012*
H5	−0.2593 (18)	1.071 (3)	0.0623 (17)	0.023 (4)*
H3	0.188 (2)	1.098 (3)	0.2325 (18)	0.039 (5)*
H4	0.036 (2)	1.148 (3)	0.2609 (19)	0.033 (5)*
H6	−0.278 (2)	0.898 (3)	0.0248 (18)	0.031 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ni1	0.00443 (5)	0.00435 (5)	0.00395 (5)	−0.00014 (4)	0.00061 (4)	−0.00003 (4)
Ni2	0.00466 (5)	0.00628 (6)	0.00601 (6)	0.00038 (4)	0.00083 (4)	−0.00025 (5)
O1	0.0087 (2)	0.0080 (2)	0.0065 (2)	−0.00071 (19)	0.00013 (18)	0.00185 (18)
O2	0.0081 (2)	0.0087 (2)	0.0066 (2)	−0.00015 (19)	0.00077 (18)	0.00260 (19)
O3	0.0065 (2)	0.0094 (2)	0.0098 (2)	−0.00125 (19)	0.00243 (18)	−0.00016 (19)
O4	0.0090 (2)	0.0106 (3)	0.0168 (3)	0.0033 (2)	0.0056 (2)	0.0040 (2)
O5	0.0071 (2)	0.0307 (4)	0.0112 (3)	−0.0001 (3)	0.0010 (2)	−0.0095 (3)
O6	0.0071 (2)	0.0090 (3)	0.0113 (3)	−0.00036 (19)	0.00169 (19)	−0.00004 (19)
C1	0.0084 (3)	0.0081 (3)	0.0084 (3)	−0.0013 (2)	−0.0006 (2)	0.0017 (2)
C2	0.0082 (3)	0.0089 (3)	0.0120 (3)	0.0016 (2)	0.0023 (2)	0.0013 (3)

Geometric parameters (Å, º)

Ni1—O1^i	2.0302 (6)	Ni2—O4^iv	2.1007 (7)
Ni1—O1^ii	2.0302 (6)	O1—C1	1.2593 (10)
Ni1—O2	2.0503 (6)	O2—C1	1.2546 (10)
Ni1—O2^iii	2.0504 (6)	O3—C2^v	1.2618 (10)
Ni1—O3^iii	2.0942 (6)	O4—C2	1.2607 (10)
Ni1—O3	2.0942 (6)	O5—H3	0.89 (2)
Ni2—O5^iv	2.0255 (7)	O5—H4	0.832 (18)
Ni2—O5	2.0256 (7)	O6—H5	0.787 (18)
Ni2—O6	2.0663 (6)	O6—H6	0.837 (18)
Ni2—O6^iv	2.0664 (6)	C1—H1	0.9500
Ni2—O4	2.1006 (7)	C2—H2	0.9500
O1^i—Ni1—O1^ii	180.00 (3)	O6—Ni2—O4	90.36 (3)
O1^i—Ni1—O2	89.49 (2)	O6^iv—Ni2—O4	89.64 (3)
O1^ii—Ni1—O2	90.51 (2)	O5^iv—Ni2—O4^iv	90.47 (3)
O1^i—Ni1—O2^iii	90.51 (2)	O5—Ni2—O4^iv	89.53 (3)
O1^ii—Ni1—O2^iii	89.49 (2)	O6—Ni2—O4^iv	89.64 (3)
O2—Ni1—O2^iii	180.00 (4)	O6^iv—Ni2—O4^iv	90.36 (3)
O1^i—Ni1—O3^iii	87.42 (2)	O4—Ni2—O4^iv	180.0
O1^ii—Ni1—O3^iii	92.58 (2)	C1—O1—Ni1^vi	120.79 (5)
O2—Ni1—O3^iii	93.27 (2)	C1—O2—Ni1	125.50 (6)
O2^iii—Ni1—O3^iii	86.73 (2)	C2^v—O3—Ni1	126.31 (6)
O1^i—Ni1—O3	92.58 (2)	C2—O4—Ni2	133.73 (6)
O1^ii—Ni1—O3	87.42 (2)	Ni2—O5—H3	121.7 (12)
O2—Ni1—O3	86.73 (2)	Ni2—O5—H4	126.1 (12)
O2^iii—Ni1—O3	93.27 (2)	H3—O5—H4	110.0 (16)
O3^iii—Ni1—O3	180.0	Ni2—O6—H5	107.5 (12)
O5^iv—Ni2—O5	180.00 (2)	Ni2—O6—H6	114.6 (11)
O5^iv—Ni2—O6	90.62 (3)	H5—O6—H6	103.0 (16)
O5—Ni2—O6	89.38 (3)	O2—C1—O1	123.70 (8)
O5^iv—Ni2—O6^iv	89.38 (3)	O2—C1—H1	118.1
O5—Ni2—O6^iv	90.62 (3)	O1—C1—H1	118.1
O6—Ni2—O6^iv	180.0	O4—C2—O3^vii	125.14 (8)
O5^iv—Ni2—O4	89.53 (3)	O4—C2—H2	117.4
O5—Ni2—O4	90.47 (3)	O3^vii—C2—H2	117.4

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O6—H5···O3^viii	0.787 (18)	1.985 (18)	2.7312 (9)	158.1 (16)
O5—H3···O2	0.89 (2)	1.87 (2)	2.7522 (9)	171.2 (19)
O5—H4···O4^viii	0.832 (18)	1.898 (18)	2.7271 (10)	174.1 (17)
O6—H6···O1^iv	0.837 (18)	1.926 (18)	2.7610 (9)	175.5 (16)

Symmetry codes: (iv) −x, −y+2, −z; (viii) −x, y+1/2, −z+1/2.