Introduction

Anhydrous nickel oxalate belongs to the isostructural family of anhydrous metal oxalates β-MeC2O4 (where Me is Fe, Co, Ni, Zn, Cu—Kondrashev et al. [1]; Cd—Jeanneau et al. [2]), with monoclinic unit cell P21/n, with similar cell parameters, arrangement of the MeO6 octahedra and oxalate anions, but different thermal decomposition to Me, MeO, or MeCO3 [3–11]. In a series of previously carried out calculations [12–18] we have applied theoretical approach based on ab initio DFT FP-LAPW calculations in order to describe the bonding properties of different anhydrous oxalates and their relation to thermal decomposition pathway, characteristic for a given compound. Since there is no detailed crystallographic data for anhydrous nickel oxalate crystal structure available (at least we were unable to find it) in order to carry out ab initio calculations of electronic structure and topological properties of total electron density allowing us to analyze electronic structure and bonding properties and providing us with additional insight into the thermal decomposition process and necessary means to explain thermal decomposition pathway in anhydrous nickel oxalate, we decided to synthesize nickel oxalate hydrate and conduct a series of XRD measurements for different temperatures to obtain necessary data.

In this paper we present the results of the synthesis, structural analysis, and theoretical studies undertaken for anhydrous nickel oxalate. The obtained results of XRD structural analysis, ab initio electronic structure calculations, bond orders, and bond valences are discussed in the light of the thermal decomposition process.

Synthesis and XRD analysis

Nickel oxalate dihydrate was prepared by precipitation from water 0.05 M solutions of oxalic acid and nickel nitrate. During precipitation, solution was stirred vigorously. pH mixture 4.0 ± 0.1 was mentioned with ammonia and hydrochloric acid to avoid precipitation of nickel hydroxide. The mixture was kept at about 60 °C for 4 h in order to salt powder aging. The crystalline powder was filtered, washed with dilute oxalic acid and next with an absolute alcohol, and finally dried at 80 °C on air. All used reagents were of analytical grade Polskie Odczynniki Chemiczne S.A.

The obtained samples were used in XRD measurements applying Philips X’Pert Pro MD diffractometer equipped with MRI high temperature cell. Standard Bragg–Brentano geometry was applied with Kα1 monochromatic beam from Cu anode. All measurements were performed in air with the 0.008 ° step size in the 15–60 ° scanning range and the 48 s of measurement time for each step. Powder sample was placed directly on the platinum strip heater; therefore, reflections from Pt around 40 and 46.5 ° are present in the collected data. The platinum strip was resistively heated at the speed of 15 K min−1 and the desired temperature was stabilized for 15 min before each measurements. Temperature was measured by Pt–Ir thermocouple spot welded to the other side of the heater. The PID controller was used in order to avoid overshooting of the desired temperature thus excluding hysteresis effect. Structural as well as qualitative and quantitative phase analyses were made by Phillips X’Pert HighScore Plus version 2.1 software with implemented full-pattern fit by means of Rietveld method.

In the Fig. 1 the contour plot for series of XRD measurements vs temperature are presented. The reflections intensity are represented by shades of gray. Clear history of NiC2O4·H2O decomposition into NiC2O4 and finally into NiO is visible. Peaks around 40 an 46.5 ° visible as a straight lines are artifacts from Pt heater. Along temperature axis, there are two distinct changes in number and reflection position. The first change, close to 200 °C takes place when C2/c phase of hydrated oxalate changes into P21/c phase of pure oxalate. Second transition, is the decomposition of oxalate into cubic form NiO around 300 °C. It is worth to note the fact of coexistence of phases at relatively wide temperature regions of about 20–50 °C (it would not be surprising in fast measurements, but in case of XRD the single scan took over 50 min including the time needed for temperature stabilization).

Fig. 1
figure 1

Contour plot for XRD measurements versus temperature

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The XRD data collected at three temperatures: 30, 300, and 365 °C, for three distinct phases, are shown in Fig. 2. Black solid line represents full-pattern fit for each phase. There are no crystallographic data for NiC2O4·H2O as well as for NiC2O4 in the literature, in order to perform full-pattern fit. Nevertheless, we have found structural similarities with other, cobalt oxalate—CoC2O4. On a basis of the structural model for CoC2O4·H2O and CoC2O4, the Co ion was substituted with Ni ion. That gave reasonable fitting result with structural details gathered in Table 1. Broad peaks for NiC2O4 as well as for NiO imply small crystallite sizes around 100 nm. Diffraction patterns also exhibit effects from preferred (111) orientation of the crystallites.

Fig. 2
figure 2

Diffraction pattern for chosen temperatures: 30, 250, and 365 °C, corresponding to NiC2O4·H2O, NiC2O4, and NiO phase, respectively. The intensity scale for each pattern is not normalized to each other in order to emphasize the details

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Table 1 Structural parameters for calculated full-pattern fit of XRD measurements
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Crystal structure

The crystal structure of anhydrous nickel oxalate is very similar to the structures of anhydrous zinc and cobalt oxalates [14, 15] and can be described as built from –C2O4–Ni–C2O4–Ni– chains, or alternatively as layered material, formed by cationic layers built from corner-shared NiO6 octahedra, linked together via bidentate chelating oxalate groups (Fig. 3).

Fig. 3
figure 3

Crystal structure of anhydrous nickel oxalate

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Each nickel atom is surrounded by six oxygen atoms, all belonging to oxalate groups: two O1 and four O2 atoms, forming highly elongated octahedron (the lengths of Ni–O bonds are 2.301, 2.321, and 2.604 Å; NiO6 octahedron volume is equal to 16.34 Å3 and respective angles between Ni and O bonds in octahedron are 68.31, 72.60, and 89.21 °). In each nickel octahedron, all four O2 atoms are shared by two octahedra belonging to neighboring chains, almost perpendicular to each other. Similarly as in case of anhydrous cobalt and zinc oxalates, one can analyze the crystal structure of Ni2C2O4 from a point of view of local environment of a given octahedron or oxalate anion: every octahedron is surrounded by four oxalate anions (Fig. 4a), each of which is in turn surrounded by four NiO6 octahedra (Fig. 4b). In Fig. 4c the oxalate anion with bonds labeled are shown, while in Table 1 the results of crystal structure parameters and in Table 2 fractional coordinates and respective Wyckoff’s positions obtained from XRD measurement for anhydrous nickel oxalate are presented.

Fig. 4
figure 4

Local environments of NiO6 octahedron (a) and oxalate anion (b) and the same anion but with bonds labels depicted (c)

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Table 2 Atomic fractional coordinates in anhydrous nickel oxalate obtained from XRD measurements
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Computational details

The ab initio calculations of electronic structure and topological properties of total electron density in NiC2O4 crystal have been carried out by means of WIEN2k FP-LAPW ab initio package [19] within density functional theory (DFT) formalism [20–25]. The following parameters has been chosen for the calculations: 500 k-points (9 × 7 × 6 mesh within the irreducible Brillouin zone), cut-off parameter Rkmax = 7.5 and GGA–PBE exchange–correlation potential [26]; the values of muffin-tin radii (R MT) (a.u.): Ni–1.8, O–1.17, C–1.17 and the convergence criteria for SCF calculations: ΔE SCF ≤ 10−5 Ry and Δρ SCF ≤ 10−5 e for total energy and electron density, respectively. The calculations were carried out with and without spin polarization in order to check the influence of spin polarization on topological properties of bond critical points. The band structure was calculated for selected path along high symmetry points defined for monoclinic unit cell (SG no. 14 on Bilbao Crystallographic Server [27–29]).

The obtained total SCF electron density distribution in crystal cell served as a basis for the calculations of topological properties of bond critical points (within Bader’s quantum theory of atoms in molecules [30] formalism). These results have been used in calculations of bond orders as defined by Pauling [31], Bader [32], Cioslowski and Mixon [33] (modified by Howard and Lamarche [34]) and bond valences, according to the Bond Valence Method (an excellent review of BVM can be found in Brown [35]). The more detailed description of methods used in present calculations can be found in our previous papers [12, 13].

Results

Electronic structure

Since nickel belongs to transition metals with partially filled d-subshell (electron configuration [Ar]3d 84s 2), for electronic structure calculations spin polarization had to be taken into account (the calculations carried out for the case without spin polarization suggested wrongly the metallic properties of anhydrous nickel oxalate). The spin polarized band structure calculated for selected path along high symmetry points with respective densities of states projected onto particular atoms is presented in Figs. 5 and 6 (for spin-up and spin-down electron density, respectively).

Fig. 5
figure 5

Band structure with band character represented by mixture of RGB colors (assigned to Ni, C, and O character, respectively; in color on-line) and partial densities of states (projected onto particular atoms) obtained for anhydrous nickel oxalate by means of DFT FP-LAPW calculations for majority electrons (spin-up electron density)

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Fig. 6
figure 6

Band structure with band character represented by mixture of RGB colors (assigned to Ni, C, and O character, respectively; in color on-line) and partial densities of states (projected onto particular atoms) obtained for anhydrous nickel oxalate by means of DFT FP-LAPW calculations for minority electrons (spin-down electron density)

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Apart from obvious differences between band structures calculated for opposite spin electron densities, there are clear similarities (also with the respective band structures obtained for different d-electron metals anhydrous oxalates presented in our earlier papers [12–15, 17, 36]) nickel 3d electrons populate mostly valence bands close to Fermi energy. Despite the fact that as in the case of zinc [14] the lack of deeper lying d sub-shells with the same symmetry results in not so effective screening of 3d electrons as in case of silver or cadmium [12, 36], due to spin polarization resulting in effective “pushing out” of minority spin-down electrons toward the higher energies (Figs. 5, 6) and in the same time higher energy dispersion comparing to majority spin-up electrons, the resulting nickel 3d atomic orbitals are more diffuse than those in zinc and their size look like respective ones in cadmium. As a result, the respective metal–oxygen bonds are longer in anhydrous nickel oxalate than in anhydrous zinc oxalate (these lengths, when no other effects e.g., geometrical ones are present, depends practically entirely on minimization of the energy due to the maximization of respective overlap integrals calculated for electron density localized in the region between respective atoms i.e., on the degree of metal 3d and oxygen 2p orbitals overlapping).

More detailed analysis of band structure and densities of states projected onto particular atomic orbitals (Fig. 7) shows that nickel majority (spin up) 3d electrons (Fig. 7a) fill mostly narrow bands with energies from −4 eV up to Fermi energy, while minority electrons fill valence bands lying close to Fermi energy and few others lying in the energy range corresponding to energy gap in case of electronic structure for majority electrons. Nickel 4s electrons (when in excited state) fill mainly bands in conduction band at energies about 5 eV. In case of carbon and oxygen atoms the effects due to spin polarization are negligible (Fig. 7b, c, d). Partial densities of states obtained for oxygen atoms O1 and O2 show, similarly as in anhydrous zinc oxalate, different patterns of states occupation—2p electrons in O1 oxygen atom fill quite uniformly bands with the energies between −7 eV and Fermi energy and too much lower extent very narrow, deeper lying bands with energies about −18 i −21 eV and also (when in excited state) to some extent bands lying conduction band. In case of O2 oxygen atom, 2p electrons, besides similarities to 2p electrons in O1 oxygen atom for bands with the energies −18 i −21 eV and conduction bands, also indicate essential difference, namely the distinct asymmetry in band occupation for bands lying close to Fermi energy (much higher share in bands occupation in energy range between −3 eV and Fermi energy and relatively small one in occupation of bands with energy in a range −7 to −3 eV). 2s electron of O1 oxygen atoms fill mostly well localized bands with the energy about −21 eV and also, too much smaller extent, bands with energy −18 and −10 eV and bands in energy range from −7 to −5 eV, whereas 2s electrons of O2 oxygen atoms dominate in bands with the energy −18 eV and to smaller extent bands with energy −21 eV, and also—like 2s electrons of O1 atoms—bands with the energy about −10 eV and within energy range from −7 to −5 eV. Carbon 2s electrons fill the same narrow bands as oxygen electrons (−21, −18, −10, −7 eV), and 2p electrons, besides the same narrow bands as 2s electrons, fill additionally bands close to Fermi energy (−7, 0 eV).

Fig. 7
figure 7

Densities of states projected onto particular atomic orbitals, obtained for anhydrous nickel oxalate by means of DFT FP-LAPW calculations (positive and negative values correspond to spin-up and spin-down electron density, respectively)

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From the results presented above, the following picture of bonding in anhydrous nickel oxalate emerges: weak nickel—oxygen bonds are formed as a result of the interactions between nickel 3d and oxygen 2p electrons. Carbon 2s and one 2p electrons fill hybridized carbon orbitals (sp 2 hybridization) and form 3 σ bonds with two oxygen atoms and second carbon atom within oxalate anion, while the second carbon 2p electron forms conjugated π* bond with two neighboring oxygen atoms.

Electron density topology

The results of topological analysis of total non spin polarized and spin polarized electron density in anhydrous nickel oxalate (obtained from DFT FP-LAPW calculations), carried out according to Bader’s QTAiM formalism are presented in Tables 3 and 4, respectively. QTAiM total charges calculated by means of CRITIC [37] program for nickel, carbon, and oxygen O1 and O2 pseudoatoms are equal to +1.070, +1.062, −0.769, and −0.828, respectively.

Table 3 Bond critical points (BCP) properties in anhydrous nickel oxalate crystal calculated for DFT FP-LAPW total non spin polarized electron density: bond length R, Hessian matrix eigenvalues λ 1–λ 3, electron density ρ BCP(r), Laplacian ∇2 ρ(r), ellipticity, electrostatic potential V(R), electronic He[ρ(r)] and kinetic G[ρ(r)] energies and bond orders (Cioslowski and Mixon formula [33], modified by Howard and Lamarche [34])
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Table 4 Bond critical points (BCP) properties in anhydrous nickel oxalate crystal calculated for DFT FP-LAPW total spin polarized electron density: electron density ρ ↑←←(r) and ρ ↓(r), Laplacian ∇2← ρ ←↑(r) and ∇2 ρ ↓(r), electrostatic potential V[ρ ↑(r)] and V[ρ ↓(r)], and electronic energy H e[ρ ↑(r)] and H e[ρ ↓(r)]
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Detailed analysis of the obtained results show that despite the necessity of taking spins into account in DFT FP-LAPW calculations, all bonds can be divided into three distinct groups (identically as in case of other anhydrous d-electron metal oxalates, analyzed earlier): (a) metal–oxygen bonds (small value of electron density at bond critical point ρ BCP(r), positive value of electron density Laplacian ∇2 ρ(r)—properties characteristic for closed shell interactions, weak ionic-covalent bonds with strong ionic character), (b) carbon–oxygen bonds (large values of electron density at BCP and strongly negative values of Laplacian–interactions with shared density, localized in bonding region; polar bonds with dominating covalent character), and (c) carbon–carbon bonds (relatively large values of electron density at BCP and strongly negative values of Laplacian—interactions with shared density, localized in bonding region; characteristic for oxalates covalent bonds, weaker than carbon–oxygen bonds).

It follows from the results presented in Tables 3 and 4 that strong asymmetry of the NiO6 octahedra do not influence in distinct way the character and properties of bond critical points, which despite the similarities to the properties of BCPs in anhydrous zinc oxalate [14] (clear asymmetry of the properties of carbon–oxygen bonds, but not so significant as in ZnC2O4 and almost no such asymmetry in case of metal–oxygen bonds) are much more like the respective BCPs in anhydrous cadmium oxalate [13]. One can see that C–C bond (r 6) in anhydrous nickel oxalate is the weakest one, Ni–O bonds are stronger than C–C bonds, but weaker than C–O bonds (r 5 bond is only slightly stronger than Ni–O bonds, but due to the much deeper potential well in the proximity of BCP for this bond in comparison with Ni–O bonds and thus more negative total electronic energy, this bond should be energetically more durable than r 1–r 3 bonds).

The data collected in Table 4 show also that the properties of the electron density at bond critical points are significantly independent on spin polarization. This conclusion has been confirmed by the calculations we have carried out for non spin polarized density and comparison of the properties of BCPs for total non spin polarized density and total spin density. The calculations showed that respective BCP properties are almost identical, especially in a case of BCPs calculated for C–O and C–C bonds. In the case of Ni–O bonds, the differences concern mostly the positions of BCPs and values of the parameters characterizing their properties, but do not change at all the overall picture of the topological properties of this compound. The lack of significant quantitative and qualitative differences between the topological parameters of electron densities obtained in calculation with and without spin polarization stems from the fact that changes in total electron density distribution due to spin polarization are more significant mostly close to nickel atoms while being much less important for the rest of the atoms (Fig. 8), and in the proximity of BCPs these differences are practically negligible. The polarization of 3d electrons, due to their odd number causes weaker screening of majority electrons, thus they feel stronger effective core potential and are concentrated closer to nickel cores, while minority electrons, being better screened are more diffused (Fig. 9) and this is a reason why mostly these electrons take part in bond formation with oxygen atoms, forcing loosening of the structure related to the necessity of preserving of such distances from oxygen atoms, to get the best overlap (maximum overlap integrals) of respective nickel and oxygen orbitals.

Fig. 8
figure 8

Differential map of total electron density in anhydrous nickel oxalate crystal obtained as a result of subtraction of non spin polarized density from spin polarized one [ρ sp(r) − ρ nsp(r)] (oxalate anion and NiO6 octahedron environments are presented here only). Yellow (darker) and blue (lighter) regions correspond to positive and negative values, respectively (in color, on-line)

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Fig. 9
figure 9

Differential map of spin density obtained for anhydrous nickel oxalate ρ ↑(r) − ρ ↓(r). Yellow (darker) and blue (lighter) regions correspond to positive and negative values, respectively (in color, on-line)

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BVM calculations

Bond valences

The results of the calculations carried out according to Brown’s bond valence model for anhydrous nickel oxalate are presented in Table 5 (atom and bond valences, deviations from theoretical valence and residual strain factors—local and global structure deviation from ideal one) and Table 6 (bond valences and bond lengths—theoretical and calculated from experimental data and resulting bond strains). The following set of equations fulfilling the valence sum rule for ideal structure were defined for anhydrous nickel oxalate (following the number and type of bonds determined in topological analysis of electron density):

$$ \begin{aligned} {\mathbf{Ni}} & = {\mathbf{2r}}_{{\mathbf{1}}} + {\mathbf{2r}}_{{\mathbf{2}}} + {\mathbf{2r}}_{{\mathbf{3}}} = {\mathbf{2}}\cdot{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} + {\mathbf{2}}\cdot{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} + {\mathbf{2}}\cdot{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} = {\mathbf{2}} \\ {\mathbf{O}}_{{\mathbf{1}}} & = {\mathbf{r}}_{{\mathbf{1}}} + {\mathbf{r}}_{{\mathbf{2}}} + {\mathbf{r}}_{{\mathbf{4}}} = {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} + {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} + {\mathbf{1}}\cdot{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} = {\mathbf{2}} \\ {\mathbf{O}}_{{\mathbf{2}}} & = {\mathbf{r}}_{{\mathbf{3}}} + {\mathbf{r}}_{{\mathbf{5}}} = {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} + {\mathbf{1}}\cdot{\raise0.7ex\hbox{$2$} \!\mathord{\left/ {\vphantom {2 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} = {\text{ }}{\mathbf{2}} \\ {\mathbf{C}} & = {\mathbf{r}}_{{\mathbf{4}}} + {\mathbf{r}}_{{\mathbf{5}}} + {\mathbf{r}}_{{\mathbf{6}}} = {\mathbf{1}}\cdot{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} + {\mathbf{1}}{\raise0.7ex\hbox{$2$} \!\mathord{\left/ {\vphantom {2 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}} + {\mathbf{1}} = {\mathbf{4}} \\ \end{aligned} $$
Table 5 Bond and atomic valences (s ij and V ij, respectively) with residual strain factors d i and D, calculated from bond valence model for anhydrous nickel oxalate
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Table 6 Theoretical and experimental (from crystal structure data) bond valences s ij and bond lengths R, relative bond length deviation from ideal one ΔR/R and bonds strain factors δ calculated for anhydrous nickel oxalate
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From the results presented in Table 5 it follows that the crystal structure of anhydrous nickel oxalate is distinguished by large deviation from ideal structure predicted within BVM model (global instability index D equal to about 0.72 of valence units). Local instability indices d i are positive for all atoms in the structure which means that on the average all bonds in this structure are too long (entire structure is loosely packed), which—taking into account the optimal bond lengths according to electron density overlapping in bond regions—results in this highly strained structure. Most strained within this structure are surroundings of nickel atoms (about +1.17 valence unit, v.u.; bonds too long, much too large octahedron with respect to ideal one). The surroundings of carbon and oxygen atom O1 are quite similar and largely strained (about +0.64 and +0.77 v.u., respectively), and the least strained (but still quite significantly—about +0.32 v.u.) is the environment of O1 oxygen atom.

Bond strains

The detailed analysis of strains present in the structure provides us with the additional information about bonding properties in this structure (Table 6): all three bonds formed by nickel with oxygen atoms are definitely too long (~10–20 %), which means that they are under tensile stress. Taking into account the values of bond orders calculated from topological properties (Table 3) it becomes apparent when these bonds breaking is followed by release of significant amount of energy and structure reconfiguration. Carbon atom forms two bonds with oxygen atoms—first one slightly too short, with small compressive stress and the second one significantly too long, under the influence of tensile stress. O1 oxygen atom forms two definitely too long bonds with nickel and one slightly too short with carbon (one can thus infer that in the proximity of this atom there should be distinct tendency to promote Ni–O bonds breaking during thermal decomposition process. Second oxygen atom O2 forms only two bonds, both too long (~10 %) one with nickel and one with carbon.

Conclusions

From the results of theoretical analysis of electronic structure and bonding properties in anhydrous nickel oxalate presented in this work, the following picture emerges: from the point of view of the geometry of the system due to the electrostatic interactions, the entire structure seems to be definitely too loose, thus the tensile stresses dominate in the crystal structure, which should result in strong structure readjustment, due to the breaking of subsequent bonds during thermal decomposition process. Chemical bonds in anhydrous nickel oxalate are distinguished by typical for anhydrous d-electron metal oxalates properties—Ni–O bonds have ionic-covalent character with dominating ionic component, while C–O and C–C bonds are typical shared interaction bonds. On a basis of the results obtained for total spin density and bond valence analysis we can propose the following pathway of thermal decomposition process as the most probable one: as a first one the weakest bond in this structure, namely C–C bond (r 6) should break. Due to large stresses in the structure (mostly tensile), one can expect almost instantaneous charge flow to C–O bonds and too much smaller extent to Ni–O bonds and shortening and strengthening of all remaining bonds (except r 4 which should become slightly longer and weaker). Carbon–oxide bond (r 4) is the strongest one in the structure and it will stay that way, even after it will weaken a little bit during the structure readjustment and stress relaxation process after C–C bond breaking, while r 5 bond will strengthen to comparable or even bigger extent than Ni–O bonds. Therefore, one can expect that nickel-oxygen bonds (r 1–r 3) should remain weaker than r 5 bond and in the following steps these bonds should break (the breaking of r 2 bond results in additional strengthening of r 5 bond, since O2 atom will be bonded in this case only with carbon atom and thus this bond, due to the relaxation of tensile stress it will immediately become shorter and stronger). As a result, the most probable, suggested by theoretical analysis, sequence of bond breaking in anhydrous nickel oxalate during the process of thermal decomposition is as follows: as the first one, r 6 bond will be broken, then r 2 and subsequently r 1 and r 3 and the final products of such process will be metallic nickel and carbon dioxide—in agreement with the results of experiments [10, 38–42].