Investigación

 

Infrared and Raman Spectra (Solid State) of Diamminediiodidecadmium (II) Complex with ¹⁵N and ²H Isotopic Subtitution

 

Claudio A. Téllez*¹ and Dilson N. Ishikawa²

 

¹ Instituto de Química. Departamento de Físico-Química. Universidade Federal Fluminense (UFF). Morro do valonguinho s/n. Niteroi-Centro. Cep.: 24210-150 Rio de Janeiro-RJ-Brazil.

² Departamento de Química. Universidade Estadual de Londrina-UEL. Londrina-Pr-Brasil.

 

Recibido el 29 de febrero del 2000.
Aceptado el 16 de mayo del 2000.

 

Abstract

The Raman and infrared spectra of the Cd(NH₃)₂I₂ complex with 15N and 2H substitution have been obtained in the solid state. Based on a normal coordinate treatment as an eleven-body problem, the vibrational spectra were assigned. With exclusion of the ammine torsional vibration, the vibrational modes of the ligand, framework-coupling and skeletal frequencies have been determined. Valence force constants values f_CdN and f_CdI were compared with the analogous force constants of Cd(NH₃)₂Cl₂ and Cd(NH₃)₂Br₂. The results of the calculations are discussed in terms of the present assignment for Cd(NH₃)₂I₂ and previously reported potential constants for the Cd(NH₃)₂X₂ and Zn(NH₃)₂X₂ (X = Cl, Br, I).

Key Words: Infrared, Raman spectra. Diamminediiodidecadmium (II) isotopic complex. Force constants.

 

Resumen

Se obtuvieron en el estado sólido los espectros Raman e infrarrojo del complejo Cd(NH₃)₂I₂, con substitución isotópica 15N y ²H. El espectro vibracional se asignó con la ayuda del análisis de coordenadas normales considerando un problema de once núcleos. Con la exclusión de las vibraciones torsionales de los grupos -NH₃, se determinaron los modos vibracionales y los números de onda correspondientes a los ligantes, acoplamiento ligantes-esqueleto, y esqueleto estructural. Las constantes de fuerza de valencia f_CdN y f_CdI se compararon con constantes de fuerza análogas de los complejos Cd(NH₃)₂Cl₂ y Cd(NH₃)₂Br₂. Los resultados de los cálculos se discuten en términos de la presente asignación vibracional para Cd(NH₃)₂I₂, y en términos de las constantes de potencial reportadas previamente para los complejos Cd(NH₃)₂X₂ y Zn(NH₃)₂X₂ (X = Cl, Br, I).

Palabras clave: Infrarrojo, espectros Raman, complejo diamino diyoduro de cadmio (II), constantes de potencial.

 

In Memory to Prof. Dr. Jacobo Gómez Lara

 

Introduction

There are many scientific works concerning the infrared and Raman active vibration of coordinated amine groups [1,2], but there is not enough information about the low-frequencies framework or skeletal metal-ligand modes, specifically on the diammine complexes. As a continuation of our studies on the infrared and Raman spectra and structure of diamminedichloridezinc (II), diamminedibromidezinc (II), diamminediiodidezinc (II), diamminedichloridecadmium (II) and diamminedibromidecadmium (II) complexes with 15N and 2H isotopic substitution [3], we have investigated the infrared and Raman spectra (solid state) of normal diamminediiodidecadmium (II) and with ¹⁵N and 2H - labeled isotopomers from 70 to 4000 cm⁻¹.

Assignment of the ammine vibrational modes is straight-forward and follows the conventional pattern [1,3]. The experimental vibrational attribution of skeletal metal-ligand modes, have been made using the 15N and 2H isotopic labeled complexes, assuming that the diammine cadmium complexes have a C_2v planar symmetry, and by comparison with the analogous infrared and Raman spectra of the Cd(NH₃)₂Cl₂ and Cd(NH₃)₂Br₂. The assignment of the vibrational spectra was confirmed by means of the normal coordinate analysis.

The obtained valence force constants were compared with the values of the Zn(NH₃)₂X₂ (X = Cl, Br, I) complexes. Bond orders were also obtained.

 

Results and Discussion

Vibrational irreducible representations: Information: the Cd(NH₃)₂Cl₂ complex, has a rhombic unit cell. The space group is C_2v11 - C_mm with two formula weights per unit cell [5]. Assuming a C_2v symmetry for a geometrical structure of trans planar configuration of Cd(NH₃)₂I₂, the 3n - 6 = 27 normal vibrations after discarding two torsional NH₃-Cd frequencies (a₂ and b₁) and two out of plane skeletal bending modes (a₁) can be distributed among the symmetry species:

The observed infrared absorption's bands and the Raman shifts with the approximate assignments are given in Table 1.

Ligand vibrations: The ν(NH) stretching, δ(HNH) bending and ρ(NH₃) rocking modes are considered as characteristic frequencies in amine complexes [1,2] and their assignments are straightforward.

Metal-nitrogen and metal-halogen stretching vibrations: According to the assumed C_2v symmetry for the trans geometry of the Cd(NH₃)₂I₂ complex, the metal-nitrogen and metal-halogen stretching vibrations should be infrared and Raman active. The ν_as(CdN)(b₂) modes were observed in the spectra at 367, 355 and 349 cm⁻¹ for the normal Cd(NH₃)₂I₂ and for the 15N and 2H isotopomers, respectively. In the Raman spectra the ν_s(CdN)(a₁) vibrational modes were found at 320, 311 and 304 cm⁻¹ for the normal and for the 15N and 2H isotopic complexes. The infrared bands found at 136, 130 and 122 cm⁻¹ for the three isotopic complexes, were assigned to the ν_as(CdI)(b₁) vibrational modes. The Raman shifts at 133, 130 and 126 cm⁻¹ in the normal compound and in the two isotopomers, were assigned to the ν_s(CdI)(a₁) stretching modes. Table 2 are listed the Cd-N and Cd-I stretching frequencies together with literature values.

X-Cd-X deformations: The Raman shifts at 115, 112 and at 110 cm⁻¹ for Cd(NH₃)₂I₂, Cd(15NH₃)₂I₂ and Cd(ND₃)₂I₂ isotopic complexes were assigned to the δ(NCdI)(a₂) vibrational mode. For the b₁ and b₂ δ(NCdI) vibrational modes, we assigned tentatively the values of 90, 82 and 81 cm⁻¹ for Cd (NH₃)₂I₂, and his isotopomers. The values of 74, 74 and 71 cm⁻¹ were assigned tentatively for the b₂ δ(NCdI) vibrational modes. Table 2 shows also our approximate assignments together with another values of wavenumbers found for the Cd(NH₃)₂X₂ (X = Cl, Br) with ¹⁵N and ²H isotopic substitution. Fig. 1 illustrates the low region of the Raman spectra.

Normal coordinate analysis (NCA)

A normal coordinate analysis for the whole structure of diamminediiodidecadmium (II) was carried out to aid principally the assignment of the framework or skeletal frequencies. The molecular structure and the internal coordinates for Cd (NH₃)₂I₂ are shown in Fig. 2. Some of the geometrical parameters which describes the skeletal structure CdA₂I₂ (A = NH₃), were taken from the literature [5], other geometrical parameters were assumed: d_Cd-N = 2.10 Å; d_Cd-I = 2.90 Å; d_N-H = 1 Å; N-Cd-I angle = 90° and M-N-H angle = 109.47°.

For the Cd(NH₃)₂I₂ complexes, initial symmetry force constants values for the NH₃-ligands were transferred from the values given by Cyvin et al. [6], while the rocking force constants value was calculated from F_ii = λ_ii / G_ii, in which, G_ii means the ith element of the kinematics coefficient matrix ( it is not the kinetic energy matrix), and λ_ii = 0.589141 (ω_ii/ 1000)2. Using the point mass model (PMM) in conjunction with the isotopic shifts, the starting set of skeletal force constants was obtained. The initial set of force constants F₀ so obtained was refined by two methods to reproduce the experimental frequencies. In the least square method [7,8] a Modified General Valence Force Field (MGVFF) was used in which the off diagonal force constants pertaining to different blocks between i) NH₃-ligand and rocking vibration, ii) NH₃-ligand and framework vibrations as well as iii) framework and rocking vibrations were all constrained to zero. In the iterative autoconsistency method [8], the off diagonal force constants between the different blocks were not constrained to zero. Results also show, that there are no significant differences between the force constants obtained by the two methods. Table 3 shows the more significative set of valence force constants. The symmetry coordinates used in the calculations are given below:

The following trends were observed in the internal (valence) force constants. The force constants value for the Cd-N stretching, f_R, increases from 1.05 mdyn/Å for Cd (NH₃)₂I₂ to 1.21 mdyn/Å for Cd(NH₃)₂Cl₂. The opposite trend was observed for the Cd-X (X = Cl, Br, I) stretching force constants, f_R-. The values goes from 0.86 mdyn/Å to 0.48 mdyn/Å. For the Zn(NH₃)₂X₂ (X = Cl, Br, I) complexes. The f_R-= f_ZnX force constants increases from 0.78 to 1.15 mdyn/Å from Zn(NH₃)₂ Cl₂ to Zn(NH₃)₂I₂ [9,10]. Structural differences also exist between the Zn(II) and Cd(II)-diamminedihalogenide complexes. The Zn (II) diamminedihalogenide one's are tetrahedral and the Cd(II) diamminedihalogenide have square-planar structures. The observed trend in the Cd-X (X = Cl, Br, I) force constants related to the Zn-X force constants can be explained as: 1) When X = Cl, we observed that fCd-Cl = 0.48 mdyn/Å and fZn-Cl = 1.15 mdyn/Å. This trend can be explained by the effect of the reduced mass for the differents oscillators M-X: (Cd-Cl = 26.95 and (Zn-Cl = 22.97, resulting from the almost 72% higher atomic mass of Cd compared with the Zn atomic mass. It is well know that when the reduced mass in a particular bond such as C-C, C-Si, C-Ge, C-Sn and C-Pb, increases, the force constants decreased. 2) When X = I, we observed that f_Cd-I = 0.86 mdyn/Å and f_Zn-I = 0.78 mdyn/Å. This trend can be explained through the different Pearson classification of Lewis acids [12]; Zn+2 is in the border line between hard and soft acids; Cd+2 is a soft acid. F- and Cl- are hard Lewis bases, and I- is a soft base. The general rule is: hard acids are those acids that react preferentially with hard bases, whereas soft acids are those that react preferentially with soft bases. In our case, Cd+2 react preferentially with I- (soft acid + soft base). The reaction between Zn+2 with I- being a reaction between an acid of the border line with a soft base. We do not have the K_ps0 values for the Cd(NH₃)₂I₂ and Zn(NH₃)₂I₂ complexes, but, for the above reasons we think that Zn(NH₃)₂I₂ has higher solubility than Cd(NH₃)₂I₂.

It is well known that the force constants for a particular bond increases with the bond order (for C₂H₂, C₂H₄ and C₂H₆, the bond orders for CC are 3.00, 2.12 and 1.11, respectively), magnitude which can be considered as a relative measure of the electronic cloud effective in holding the two atoms together, and represents the effective number of covalent (electron pair) or electrovalent (ionic) bonds acting between the two atoms considered [11]. As the normal coordinate has revealed, there are not coupling between the skeletal ν(CdN) and ν(CdI) normal modes, then as an approach the Cd-N and Cd-X bond order were calculated by the Gordy's equation [11]

where F is the Cd-N force constant, N is the bond order, d is the Cd-N bond length, and X_cd and X_N are the Cd and N atom eletronegativities according to Allred and Rochow [13] and Mulliken [14].

Table 4 gives the bond order of the Cd-N and Cd-X bonds with X = Cl, Br in the Cd(NH₃)₂Cl₂, Cd(NH₃)₂Br₂ and Cd(NH₃)₂I₂ complexes, these values reflect the influence of the halogenide ligand on the square planar coordination of Cd+2 cation, and indicate rather little covalent degree.

The potential energy distribution reveals that the vibrational modes pertaining to the ligands and framework coupling are nearly pure (values higher than 95%).

 

Experimental

The complexes Cd(NH₃)₂I₂, Cd(15NH₃)₂I₂ and Cd(ND3)₂I₂ were prepared in mg quantities using the suggestion given by Perchard and Novak [4]. The samples as nujol mulls are on polyethylene supports. The infrared spectra from 4000 to 40 cm⁻¹ were run with a Nicolet 60 SXB Fourier transform infrared (FT-IR) spectrometer equipped with a DTGS detector with polyethylene windows. For the IR measurements the resolution was 4 cm⁻¹ in the 4000 - 1500 region, 2 cm⁻¹ between 1500 and 700 cm⁻¹ and 0.25 cm⁻¹ in the range from 700 to 40 cm⁻¹. The Raman spectra were recorded on a Jarrel-Ash 25-300 double grating monochromator with an RCAC31034A photon counting system. The resolutions used in the measurement were: 5 cm⁻¹ in the spectral range from 3500 to 2000 cm⁻¹ and 2 cm⁻¹ in the range from 2000 to 20 cm⁻¹. The measurement was carried out using 514.5 nm radiation from an Ar+ laser. The samples were sealed in capillary tubes.

 

Acknowledgments

The authors thanks the financial assistance of FINEP/PADCT, process number Nº 6595038100 and the CNPq (research grant).

 

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