High-co-ordination-number compounds of niobium and tantalum: reactions of niobium and tantalum halides and sulphide-halides with sodium diethyldithiocarbamate. The crystal structures of [Nb(S2CNEt2)4]Br, Nb(S2CNEt2)3S, and Ta(S2CNEt2)3(S2)

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Abstract
The reactions of Na(S2CNEt2) with niobium(V) halides lead to the formation of a mixture from which [Nb(S2CNEt2)4]Br, (1), and Nb(S2CNEt2)3S, (2), can be separated. The corresponding reactions with tantalum sulphide-halides gave Ta(S2CNEt2)3S and Ta(S2CNEt2)3(S2), (3). The products (1)–(3) have been characterised by single-crystal X-ray structure determinations: (1), monoclinic, space group P21/a, a= 11.100(8), b= 27.910(18), c= 10.680(9)Å, β= 92.8(1)°, and Z= 4; (2), triclinic, space group P, a= 16.892(14), b= 9.868(8), c= 15.747(14)Å, α= 91.0(1), β= 78.1(1), γ= 85.8(1)°, and Z= 4; (3), monoclinic, space group C2/c, a= 16.319(9), b= 10.887(8), c= 30.322(25)Å, β= 88.0(1)°, and Z= 8. For compound (1), 2 612 above background data were collected on a diffractometer and the structure refined to R 0.081; for (2) and (3) the corresponding values are 5 058, R= 0.054 and 1 814, R= 0.099 respectively. The structure of (1) is ionic and contains the [Nb(S2CNEt2)4]+ cation in which the metal is eight-co-ordinate with mmmm dodecahedral symmetry. The Nb–S distances range from 2.511(5) to 2.593(4)Å. The structure of (2) comprises two crystallographically distinct seven-co-ordinate monomers of Nb(S2CNEt2)3S of similar geometry, both with a multiple NbS bond [2.164(3) and 2.112(3)Å] and three chelating S2CNEt2 ligands. In both molecules the metal geometry is pentagonal bipyramidal with the multiple bond in an axial position. The structure of (3) contains discrete molecules of Ta(S2CNEt2)3(S2) which are eight-co-ordinate with the metal in a distorted dodecahedral environment. Four m edges are occupied by three S2CNEt2 ligands and a side-on bonded disulphide group. The Ta–S distances range from 2.375(12) to 2.598(10)Å with a S–S distance of 2.074(17)Å.

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