**19.1. Electronic spectra of nickel compounds**

The electronic distribution of Ni(II) ion (d8) is 6 2 <sup>2</sup>*g g t e* which gives rise to 3F, 3P, 1D and 1S terms of which 3F is the ground state. In a cubic crystal, these terms transform as follows:

$$\begin{aligned} \,^3F &\rightarrow \,^3T\_{1\_\mathcal{S}}\left(F\right) + \,^3T\_{2\_\mathcal{S}}\left(F\right) + \,^3A\_{2\_\mathcal{S}}\left(F\right) \\\\ &\stackrel{\cdot}{\,}^3F \rightarrow \,^3T\_{1\_\mathcal{S}}\left(P\right) \\\\ &\stackrel{\cdot}{\,}^1D \rightarrow \,^3T\_{2\_\mathcal{S}}\left(D\right) + \,^1E\_\mathcal{s}\left(D\right) \\\\ &\stackrel{\cdot}{\,}^1G \rightarrow \,^1T\_{1\_\mathcal{S}}\left(G\right) + \,^1T\_{2\_\mathcal{S}}\left(G\right) + \,^1A\_{1\_\mathcal{S}}\left(G\right) \\\\ &\stackrel{\cdot}{\,}^1S \rightarrow \,^1A\_{1\_\mathcal{S}}\left(S\right) \end{aligned}$$

Of these crystal field terms, 3A2g(F) is the ground state. Hence three spin allowed transitions are possible and the others are spin forbidden The three spin allowed transitions are: 3A2g(F) → 3T1g(P), 3A2g(F) → 3T1g(F) and 3A2g(F) → 3T2g(F). These transitions are governed by linear equations as given below:

$$\prescript{3}{}{A}\_{2\_{\mathcal{S}}}\left(F\right) \xrightarrow{\sim} \prescript{3}{}{T}\_{1\_{\mathcal{S}}}\left(P\right) = 15Dq + 7.5B + 6B\left(1 + \mu\right)\Big/\frac{\mathcal{Y}}{2} = \nu\_{1} \tag{34}$$

$$\sideset{}{^3A}{}{\mathop{A}}\_{2g}\left(F\right)\to\sideset{}{^3T}{}\_{1g}\left(F\right)=15Dq+7.5B-6B\left(1+\mu\right)\lambda^2=\nu\_z\tag{35}$$

$$\sideset{^3A}{\_{2}}{\_{3}}(F)\rightarrow\ \sideset{^3T}{\_{2}}{\_{3}}(F)=10D}q=\nu\_{\s}\tag{36}$$

Here μ is of the order of 0.01. Dq and B are of similar magnitude. The spin allowed bands are calculated using the above equations whereas the spin forbidden bands are assigned using Tanabe-Sugano diagrams.

#### **19.2. Typical examples**

36 Advanced Aspects of Spectroscopy

Dq= 930, B= 600 and C =2475 cm-1 ,α= 90 cm-1

12100

15270

23380

Wave number (cm-1) Wave

793

608 585 427

coloring of glass to which it gives a green hue.

**19.1. Electronic spectra of nickel compounds** 

The electronic distribution of Ni(II) ion (d8) is 6 2

length (nm)

793

608 585 427

**Table 15.** Band headed data with assignments for Fe(III) in various compounds

Wave length (nm)

827

655 430

**19. Nickel** 

Prehnite Plumbojarosite Transition

length (nm)

Dq= 900, B= 700 and C =2800 cm-1 α= 90 cm-1

<sup>2</sup>*g g t e* which gives rise to 3F, 3P, 1D and 1S terms

Wave number (cm-1)

4T1g(G)

4T2g(G)

4A1g(G), 4E(G)

4T2g(D)

4Eg(D)

from Site I Site II 6A1g

Wave number (cm-1) Wave

12528

16331

23276

Nickel is the 7th most abundant transition metal in the earth's crust. The electronic configuration of nickel is [Ar]4S23d8. Nickel occurs in nature as oxide, silicate and sulphide. The typical examples are garnierite and pentlandite. Nickel exhibits +1 to +4 oxidation states. Among them divalent state is most stable. Nickel compounds are generally blue and green in color and are often hydrated. Further, most nickel halides are yellow in color. The primary use of nickel is in the preparation of stainless steel. Nickel is also used in the

of which 3F is the ground state. In a cubic crystal, these terms transform as follows:

 33 3 3 12 2 *gg g F TF TF A F*

> 3 3 <sup>1</sup>*<sup>g</sup> F TP*

 13 1 *D T D ED* <sup>2</sup>*g g*

 11 1 1 1 *G T G T G EG A G* 1 2 *g gg g*<sup>1</sup>

> 1 1 <sup>1</sup>*<sup>g</sup> S AS*

Observed Calculated Observed Calculated Observed Calculated

Dq= 900, B= 600 and C =2500 cm-1 α= 90 cm-1

12610

16445 17095 24390

> The data chosen from the literature are typical and representative for each sample. For more complete information on any specific case, original references are to be consulted. X-band spectra and optical absorption spectra of the powdered sample are recorded at room temperature (RT) only.

> *Divalent Nickel [d8]:* The optical absorption spectrum of falcondoite mineral recorded in the mull form at room temperature shows three intense bands at 9255, 15380 and 27390 cm-1 and a weak band at 24385 cm-1. Using the equations 34 to 36, the calculated values of Dq and B are 925 and 1000 cm-1 respectively. Using these Dq and B values and T-S diagrams for d8 configuration, the cubic field energy matrices and Racah parameter, C are evaluated (4.1B) .

> Ni2+ also gives absorption bands in the NIR region. These bands suggest that Ni2+ is in tetrahedral site. In some of the samples, Ni2+exbits both octahedral and tetrahedral coordination. Several examples are available in the literature. Some of them are given in the Table-16.


**Table 16.** Assignment of bands for Ni(II) with 3A2g(F) as the ground state. All values are given in cm-1.
