Assignment of oxidation states of metal ions in Zinc and Cadmium dithiocarbamate complexes and their adducts

INTRODUCTION

The bond valence sum method is popular method in coordination chemistry to estimate the oxidation states of atoms. This method relates the bond lengths around a metal center to its oxidation state. Historically from the concept of bond number, this method was originally propounded by Pauling¹. Later I. D. Brown and other scientists further fleshed it out^2-6. The advantage of this approach is that the bond length is a unique function of bond valence. Generally for a particular bond type, the bond valence diminishes exponentially as the bond length increases. In this semi-empirical method, the valence ‘v’ of a bond between two given atoms i and j is related by an empirical relation

V_ij = exp [(R_ij-d_ij/B)].                                                             (1)

where d_ij is the bond distance in Å and R_ij is a parameter characteristic of the bond. Like d_ij, R_ij is known as the bond valence parameter with the same unit in Å. R_ij parameter is coordination number and geometry specific. Here ‘B’ is considered as universal constant, which is equal to 0.37. The oxidation number Vi of the atom i is simply the algebraic sum of these ‘v’ values of all the bonds  around the atom i, following equation

Σvij = V_{i.                                                                                          (2)}

This V_i is known as the BVS of the ith atom. Thus if R_ij is known for a particular bond type, the BVS can be calculated from the crystallographically determined d_ij values.   The R_ij parameters reported by two groups of authors are used in the present calculations. R_ij(OK/B) is defined as ⁶:

R_ij = r_i+r_j-[r_ir_j(√c_i-√c_j)²]/[c_ir_i+c_jr_j]

where r_i and r_j are  size parameters of the atom i and j involved in bonding and c_i, c_j are additional parameters associated with atoms i and  j such that R_ij = r_i+r_j-(c_i,c_j,r_i,r_j) and if i = j then f = 0.  R_ij(B/OK) values reported in references⁴,  have also been used in the present calculations. In the case of mixed ligand complexes involving nickel-dithiocarbamates and phosphorous donor ligands, the BVS values are higher than the expected formal oxidation state of +2 due to the back bonding effect associated with the Ni-P distance^7,8.  But for the divalent zinc and cadmium dithiocarbamte complexes the BVS value results in excellent agreement with the formal oxidation state of the metal^9,10. In continuation of our interest in assigning oxidation states on metals in metal  dithiocarbamate complexes, in this work the crystallographic distances for a series  of  zinc and cadmium complexes and their adducts  have been  collected from the literature and  the formal oxidation state of the metal ion were assigned by using BVS method.

RESULTS AND DISCUSSION

Calculations involving various parameters to determine R_ij(OK/B), R_ij(B/OK) for the listed complexes and a representative calculation of BVS values are given in Table 1 and 2 respectively. The bond valence sums (BVS) of zinc and cadmium complexes are given in Table 3 and 4.

Table 1: Size parameters

Bond	Rij(OK/B)	Rij(B/OK)
Zn-S Zn-N Cd-S Cd-N	2.08 1.77 2.28 1.96	2.09 1.77 2.29 1.96

 

By making use of two different sets of parameters such as V_i(OK/B)  and V_i(B/OK) the bond valence sums are calculated.  Results of the investigations clearly showed  the BVS values to be close to’2’ which is equivalent to the formal oxidation state of zinc in the zinc complexes considered. The latter value, V_i(B/OK),  shows better agreement than the former with respect to the formal oxidation state of the central ion. Therefore both the BVS values V_i(OK/B) and V_i(B/OK) are close to 2.0 indicating the valence of the zinc in the complexes, irrespective of the coordination number.

Table 2: BVS values for [Zn(S₂CN(Me)ⁱPr)₂]

Bond	dij	vij(OK/B)	vij(B/OK)
Zn-S Zn-S Zn-S Zn-S Zn-S	2.339 2.432 2.334 2.902 2.365	0.497 0.387 0.503 0.109 0.468	0.511 0.387 0.517 0.112 0.475
	Vi =	1.964	2.002

 

Change in coordination number and change in coordination environment  around the zinc ion in the complexes have adjusted themselves in such a way that the valency of the central ion is satisfied. Generally, the valence bond sums for the parent zinc dithiocarbamate complexes are greater than the sums of the adducts¹¹. In the case of the adducts, the Zn-S distances are longer than the Zn(dtc)₂  complexes due to the presence of an additional neutral ligand causes an increase of the Zn-S bond lengths. The increase in  Zn-S distances in the adducts indicates the weakening of the bond and this is very well in keeping with the BVS values obtained in the earlier studies¹¹.  In the present study also similar trends have been observed with respect to the Zn-S distances and BVS values. But, interestingly  the  bond valence sum value of [Zn(S₂CN(CH₂)₄)₂(2,9-Me₂-1,10-Phen)] adduct is significantly greater than the  value of parent  [Zn(S₂CN(CH₂)₄)₂]. The increase in BVS values can be ascribed to the decrease in Zn–N and Zn-S distances (which are significantly shorter than those observed  in    4,7-Ph₂-1,10-Phenanthroline analogue and  in other nitrogenous adducts) due to the effect of electron releasing methyl substituent on 1,10-phenanthroline.

In all the  cadmium complexes, the BVS values have been found to be close to ‘2’ irrespective of the coordination number, which confirm the valency of the cadmium in the complexes. It has already been reported¹¹ that in the case of the cadmium dithiocarbamate complexes and their adducts no observable changes are seen in BVS values. Owing to the larger size of cadmium ion when compared with zinc ion, the Cd-S distances  are not much affected by the change in coordination geometry from four coordination to five  or six. The BVS analysis for the bisdithiocarbamates of zinc and cadmium and their adducts shows the valency of the central metal to be 2.0 as expected, which confirms the correctness of the related crystal structures. However, in the process, the valence of the central ion is fulfilled and the situation justifies the statement “ formation of a complex involving metal ion and multidentate ligands represents a compromise between the steric interactions in the ligand and the steric and electronic requirements of the metal ion”.

Table 3: Bond Valence Sums for Zinc complexes

 

Compound	Coordi nation number	Vi(OK/B)	Vi(B/OK)
[Zn2(C6H12NS2)4][Zn2(C7H14NS2)4 ]                                        [Zn(S2CN(Et)Ph)2]2[Zn(S2CN(iPr)2)2]2[Zn(S2CN(CH2)4)2]2[Zn(S2CN(CH2)6)2]2 [Zn(S2CN(Me)Et)2]2 [Zn(S2CN(Me)nPr)2]2 [Zn(S2CN(Me)iPr)2]2 [Zn(S2CN(Me)nBu)2]2 [Zn(S2CN(Me)Ph)2]2 [Zn(S2CN(Et)iPr)2]2 [Zn(S2CN(Me)Cy)2]2 [Zn(S2CN(Et)Cy)2]2       [Zn(S2CNCy2)2]2 [Zn(S2CN(Et)2(C5H5N)] [Zn(S2CNMe2)2 (C5H11N)] [Zn(S2CN(CH2)4)2(2,2’-bipy)] [Zn(S2CN(Me)Cy)2(2,2’-bipy)] [Zn(S2CN(CH2)4)2(2,9-Me2-1,10-Phen)] [Zn(S2CN(CH2)4)2(4,7-Ph2-1,10-Phen)] [Zn(C9H18NS2)2(1,10-Phen)]                                 [Zn2(S2CNEt2)4(trans-NC5H4C(H)=C(H) C5H4N)] [Zn(S2CN(Me)iPr)2 (1,10-Phen)]	4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 5 5 6 6 6 6 6 5 6	1.89 1.91 1.90 1.94 1.90 1.95 1.94 1.97 1.96 1.94 1.91 1.89 1.96 1.92 2.01 1.95 1.82 1.87 1.85 2.00 1.92 1.84 1.92 1.89	1.95 1.96 1.95 2.00 1.95 2.01 2.00 2.03 2.00 1.98 1.96 1.95 2.02 1.97 2.07 1.99 1.85 1.90 1.89 2.03 1.96 1.87 1.96 1.93

^a Actual coordination number is five including a long Zn-S bond.

Table 4:  Bond Valence Sums for Cadmium complexes.

 

 

^aActual coordination number is five including a long Cd-S bond.

(CH₂)₄CNS₂^– = pyrrolidinedithiocarbamate anion, C₆H₁₂NS₂^– = N-ethyl-N-isopropyldithiocarbamate anion, ^–S₂CNCy₂=N,N-dicyclohexyldithiocarbamate anion, ^–S₂CN(Et)Ph = N-ethyl-N-phenyl dithiocarbamate anion, ^–S₂CN(Et)₂ = N,N-diethyldithiocarbamate anion, ^–S₂CN(ⁱPr)₂ = N,N-diisopropyl dithiocarbamate anion, ^–S₂CN(Me)ⁱPr= N-isopropyl-N-methyldithiocarbamate anion, ^–S₂CN(Me)Et = N-ethyl-N-methyldithiocarbamate anion, C₇H₁₄NS₂^– = N-butyl-N-ethyldithiocarbamate anion, ^–S₂CN(Me)Bu= N-butyl-N-methyldithiocarbamate anion, ^–S₂CN(Me)Ph=N-methyl-N-phenyldithiocarbamate anion,  ^–S₂CN(Me)Cy = N-cyclohexyl-N-methyldithiocarbamate anion, ^–S₂CN(ⁱBu)₂=   N,N-di-isobutyldithiocarbamate anion,^–S₂CN(nPr)₂ =  di-n-propyldithiocarbamate anion, ^–S₂CN(Bz)₂ = N,N-dibenzyldithiocarbamate anion, ^–S₂CN(Bz)(CH₂CH₂OH)= N-(2-hydroxyethyl)-N-benzyl dithiocarbamate anion, S₂CN(Et)Cy = N-cyclohexyl-N-ethyldithiocarbamate anion, ^–S₂CN(Et)ⁱPr=N-isopropyl-N-ethyldithiocarbamate anion, ^–S₂CNMe₂  = N,N-dimethyldithiocarbamate anion,  C₉H₁₈NS₂^–=N,N-di-n-butyldithiocarbamate anion, (CH₂)₆CNS₂^–= hexamethylenedithiocarbamate anion, 2,2’bipy  =  2,2’-bipyridine,  C₅H₁₁N = piperidine 1,10 – Phen =1,10-phenanthroline, C₅H₅N = Pyridine, NC₅H₄C(H)=C(H)C₅H₄N =  bis(4-pyridyl)ethylene. The fragment C₃H₅ is –CH₂C(H)=CH₂, i.e. allyl.

CONCLUSIONS

The bond valence sum (BVS) model can be applied to determine compatibility between a given coordination model and a particular unknown oxidation state. This method is useful to assign the oxidation state of the metal ions in main group metal  complexes. In this paper, BVS have been  calculated for a series of  Zn(II) and Cd(II) dithiocarbamate complexes and their adducts. The  BVS analysis has confirmed the valency of the central metal to be 2.0 as expected.  It is concluded that the formation of a complex of any metal ion with multidentate ligand represent a compromise between the steric interactions in the ligand and the steric and electronic requirements of the metal.

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