Valence Bond Sum (VBS) Analysis on Bis(Dithiocarbamato)Nickel(II) Complexes with Nis₄ Chromophore

INTRODUCTION

With traditional use of the valence bond  sum (VBS) method, the oxidation state of a central atom can be determined if the bond valence parameter (R_ij) value and the lengths of the bonds from donor atoms to the central atom are known. The VBS can be extremely useful to all chemists in resolving conflicts regarding oxidation states or in evaluating the results of a crystal structure analysis.   The chemist wishing to estimate an unknown bond length in a molecule or crystal is confronted with an intimidating array of covalent radii, ionic radii, metallic radii etc., from which to choose¹. The bond valence method^2,3 has recently had considerable success in predicting and interpreting bond lengths in ‘ionic solids’. As it can be applied to estimate the bond lengths, vice-versa the sum of these bond lengths should give information about the valence of the central ion. Using the crystallographic data reported by our research group, the VBS calculations were made and results were published for a series of  zinc, cadmium and nickel dithiocarbamate complexes and their adducts^4-6. The VBS investigations for divalent zinc and cadmium dithiocarbamte complexes resulted in excellent agreement with the formal oxidation state of the metal. But for  the  nickel  complexes, involving nickel-dithioocarbamates and phosphorous donor ligands, the VBS values are higher than the expected formal oxidation state of +2.  In continuation of our interest in VBS calculations on metal  dithiocarbamate complexes, the present anlaysis was undertaken to improve the VBS tremendously on nickel(II)dithiocarbamates  by using the new R_ij(T) parameter. For this analysis the crystallographic distances for a series  of parent nickel dithiocarbamate complexes  have been  collected from the literature and  the VBS results are reported in this paper. The valence v_ij of a bond between two atoms i and j is defined so that the sum of all the valences from a given atom i with valence V_i obeys⁷ åv_ij = V_i. The most commonly adopted relationship for the variation of the  bond length d_ij with valence is v_ij = exp[(R_ij-d_ij/B)].  Here ‘B’ is taken to be a universal constant equal to 0.37. For inorganic compounds, including those of transition metals, the parameter B is commonly accepted^7,8  to have a value of 0.37.  The parameter R_ij is the bond valence parameter.  The R_ij parameters reported by two groups of authors are used in the present calculations. R_ij(OK/B) is defined as ⁹:

R_ij = r_i+r_j-[r_ir_j(Öc_i-Öc_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 references⁷,   have also been used in the present calculations.

RESULTS  AND DISCUSSION

Valence bond parameters, R_ij, available in the literature^7,9 for Ni–S, Ni–N, Ni-P, Ni–O are obtained from a statistical   consideration of a very large number of homoleptic extended solid⁷. Use of those R_ij values⁷, for isolated independent molecules of    metallo-organic nature yielded very high VBS values leading to erroneous conclusion. Use of R_ij values determined from homoleptic extended solids in the calculations of VBS for divalent zinc, cadmium and mercury metallo-organic compounds resulted in excellent agreement with the formal oxidation state of the metal^7,9. The observation is a  clear case  of a more or less ionic interaction prevailing in metallo-organic compounds involving d¹⁰ metal ions.      For compounds which involve  transition metal ions such as Mo, Mn, Cu, Fe, Ni the agreement of the calculated  oxidation states always are far higher than their formal oxidation states^10,11. The bond valence sums for metal ions in isolated independent metallo-organic molecules agreed well with their formal oxidation by the use of a new set of R_ij parameters^10,11.   H.Thorp reported a set of new optimized R_ij(T) parameters for Ni⁺²-O, Ni⁺²-S, Ni⁺²-N along with other data derived from isolated model compounds involving such interactions. Use of the R_ij(T) parameters for the parent bisdithiocarbamates of nickel(II) improved the VBS tremendously and the formal oxidation state of nickel is observed to be close to 2.0. Valence bond sums are calculated for a series of  complexes by making use of three different sets of parameters such as V_i(OK/B), V_i(B/OK) and Vi(T)  reported in this paper. The calculated R_ij parameters are 2.058 R_ij(OK/B), 2.04 R_ij(B/OK) and  1.937 R_ij(T). A representative calculation of VBS values and the  valence bond sums (VBS) of nickel dithiocarbamate complexes are  given in Table 1 and 2 respectively.

Table 1. VBS values for [Ni(C₇H₁₂NS₂)₂]

 

Bond	dij	vij(OK/B)	vij(B/OK)	vij(T)
Ni-S	2.1964	0.688	0.655	0.496
Ni-S	2.1923	0.696	0.663	0.502
Ni-S	2.2009	0.680	0.647	0.490
Ni-S	2.1892	0.701	0.668	0.506
	Vi =	2.765	2.633	1.994

 

Table 2.  Valence Bond Sums for Nickel dithiocarbamate complexes

 

Compound	Vi(OK/B)	Vi(B/OK)	Vi(T)
[Ni{S2CN(i-C3H7)2}2]	2.856	2.720	2.060
[Ni{S2CN(n-C4H9)(C2H5)}2]	2.700	2.572	1.948
[Ni(C7H12NS2)2]	2.765	2.633	1.994
[Ni{S2CN(C2H4OH)2}2]	2.718	2.588	1.960
[Ni{S2CN(i-C4H9)2}2]	2.725	2.596	1.965
[Ni{S2CN(C2H5)2}2]	2.720	2.590	1.960
[Ni{S2CN(C4H8O)2}2]	2.636	2.510	1.900
[Ni{S2CN(CH2CH2NEt)2}2]	2.686	2.560	1.936
[Ni(S2CNC3H6C6H4)2]	2.704	2.574	1.950
[Ni(S2CNHC10H15)2]	2.718	2.590	1.960
[Ni{S2CN(CH2CH2OMe)2}2]	2.700	2.570	1.944
[Ni(S2CNC5H10)2]	2.712	2.576	1.958
[Ni(S2CNH2)2]	2.616	2.494	1.886
[Ni(S2CNHMe)2]	2.730	2.600	1.968
[Ni{S2CN(n-C3H7)2}2]	2.704	2.574	1.948
[Ni(S2CNHMePh)2]	2.704	2.574	1.950
[Ni{S2CN(CH2)4}2]	2.682	2.554	1.934
[Ni{S2CN(n-C3H7) (C2H4OH)}2]	2.700	2.545	1.930
[Ni(C7H10NS2)2]	2.696	2.568	1.944
[Ni(C10H10NOS2)2]	2.722	2.592	1.962
[Ni(C11H22NS2)2]	2.782	2.708	2.050
[Ni(C18H34NS2)2]	2.686	2.556	1.936
[Ni{S2CNH(n-C3H7)}2]	2.716	2.586	1.958
[Ni{S2CNH(i-C3H7)}2]	2.768	2.636	1.996
[Ni(C10H10NS2)2]	3.140	2.992	2.266

 

The Crystal structure data of the complexes were obtained from the corresponding literature.

OK/B = calculated by the method due to O’Keeffee and Brese

B/OK = calculated by the method due to Brese and O’Keeffee

T   = calculated by the method due to  H. H. Thorp

S2CN(i-C3H7)2 = N,N-diisopropyldithiocarbamato anion, -S2CN(n-C4H9)(C2H5) = N-ethyl-N-butyldithiocarbamate anion, C7H12NS2-= 4-methylpiperidinecarbodithioato anion, -S2CN(C2H4OH)2= N,N-di(2-hydroxyethyl)dithiocarbamate anion,  -S2CN(i-C4H9)2 = N,N-diisobutyldithiocarbamate anion,   -S2CN(C2H5)2 = N,N-diethyldithiocarbamate anion, -S2CN(C4H8O)2 = 4-morpholinecarbodithioato anion,     -S2CN(CH2CH2NEt)2 = 2-diethylaminoethyldithiocarbamate anion, -S2CNC3H6C6H4 = 1,2,3,4-tetrahydroisoquinolinedithiocarbamate anion, -S2CNHC10H15 = N-adamantyldithiocarbamate anion,  -S2CN(CH2CH2OMe)2= bis(2-methoxyethyl)dithiocarbamate anion, -S2CNC5H10 = piperdinecarbodithioato anion, -S2CNHMe = methyldithiocarbamate anion, -S2CN(n-C3H9)2= di-n-propyldithiocarbamate anion,       -S2CNHMePh = N-Methyl-N-phenyldithiocarbamate anion, -S2CN(CH2)4 = Pyrrolidinedithiocarbamate anion, -S2CN(n-C3H7)(C2H4OH) = N-Propyl-N-(2-hydroxyethyl)dithiocarbamate anion, C10H10NOS2- =    N-acetyl-N-benzyldithiocarbamate anion, C7H10NS2- = N,N-diallyldithiocarbamate anion, C11H22NS2- = dipentyldithiocarbamate anion, C18H34NS2- = N-ethyl-N-cyclohexyldithiocarbamate anion, -S2CNH(n-C3H7)  =   N-Propyldithiocarbamate anion, -S2CNH(i-C3H7)  =   N-iso-propyldithiocarbamate anion, C10H10NS2 = 1,2,3,4-tetrahydroquinolinedithiocarbamate anion,

CONCLUSIONS

Valence bond sum (VBS) is  used by many researchers to determine the oxidation state of metal ions in solids based on crystallographically determined metal-ligand bond distances. In the  transition metal  complexes  the calculated oxidation states by using R_ij(OK/B) and R_ij(B/OK)  are always far higher than their formal oxidation states. In order  to improve the VBS tremendously on a series of  nickel(II)dithiocarbamates a new valence bond parameter, R_ij(T), is introduced and  the formal oxidation state of nickel is observed to be close to 2.0

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