Anion SSNS

A wide variety of cyclic S-N compounds containing two or three coordinate sulfur is known (i ), The binary S-N rings vary in size from four (S2N2) to ten (SsNs" ") atoms and examples of anions (SsNs") and cations (S3N2 , S N3 , S N ) are known in addition to neutral molecules (2). The excess of electrons in these planar, electron-rich heterocycles is often accommodated in low lying tt orbitals but, in some cases (e.g. S N, S Ns"), the number of tt electrons is reduced by formation of transannular S-S bonds to give cages ( , ). 

S-N compounds, for example, S2N2 (3), S4N4 (1), SsNe (8), and alkali metal salts of the anions SsNs (9)andS4Ns (10). These compounds may explode when subjected to mechanical or thermal stress, for example, on contact with a metal spatula or upon gentle heating. 

Paradoxically, although they are electron-rich, S-N compounds are good electron acceptors because the lowest unoccupied molecular orbitals (LUMOs) are low-lying relative to those in the analogous carbon systems. For example, the ten r-electron [SsNs] anion undergoes a two-electron electrochemical reduction to form the trianion [SsNs] whereas benzene, the aromatic hydrocarbon analogue of [SsNs], forms the monoanion radical [CeHg] upon reduction. ... 

The orange-red [SsN] anion Xm2.x 465 nm) is obtained by the addition of triphenylphosphine to a solution of a [S4N] salt in acetonitrile.It can be isolated as a salt in combination with large counterions, e.g., [Ph4As] or [N(PPh3)2] , but it is unstable with respect to the formation of the blue [S4N] anion in solution or in the solid state under the influence of heat or pressure. 

Although the structure of [SsN] has not been established by X-ray crystallography, the vibrational spectra of 30% N-enriched [SsN] suggest an unbranched [SNSS] (5.22) arrangement of atoms in contrast to the branched structure (Dsh) of the isoelectronic [CSs] and the isovalent [NOs] ion (Section 1.2). Mass spectrometric experiments also support the SNSS connectivity in the gas phase.Many metal complexes are known in which the [SsN] ion is chelated to the metal by two sulfur atoms (Section 7.3.3). Indeed the first such complex, Ni(S3N)2, was reported more than twenty years before the discovery of the anion. It was isolated as a very minor product from the reaction of NiCl2 and S4N4 in methanol. However, some of these complexes, e.g., Cu and Ag complexes, may be obtained by metathetical reactions between the [S3N] ion and metal halides. 

NMR and visible spectra have established that a number of S-N anions are present in such solutions.The primary reduction products are polysulfides Sx, which dissociate to polysulfur radical anions, especially the deep blue 83 ion (/Imax 620nm). In a IM solution the major S-N anion detected by NMR spectroscopy is cycZo-[S7N] with smaller amounts of the [SSNSS] ion and a trace of [SSNS]. The formation of the acyclic anion 5.23 from the decomposition of cyclo-Sjl is well established from chemical investigations (Section 5.4.3). The acyclic anions 5.22 and 5.23 have been detected by their characteristic visible and Raman spectra. It has also been suggested that a Raman band at 858 cm and a visible absorption band at 390 nm may be attributed to the [SaN] anion formed by cleavage of a S-S bond in [SSNS]. ° However, this anion cannot be obtained as a stable species when [SsN] is treated with one equivalent of PPhs. 

Complexes of the [S3N]- anion are obtained in good yields via deprotonation of CJC/0-S7NH in the presence of metal halides or by using mer-cury(II) complexes [Hg(S7N)2] or [HgPh(S7N)] as a source of [S3N]. The [SSNS] ligand is invariably chelated to the metal via the two terminal sulfur atoms. 

The monoanion (SSn = —30.3) and dianion (5Sn = 186.7) of the phenylstannoles have been prepared through the bis(stannole) as their lithium derivatives as shown in Scheme 7.237 238 The l-t-butyl-2,3,4,5-tetraphenylstannole anion is oxidized in air to the corresponding bis(stannole).239... 

Although cyclic structures are unhkely for the sulfur-rich anions, S4N and SsN, because of ring strain, it should also be noted that the tc and 7t levels would be fully occupied in planar rings. In fact, the anions adopt open chain structures in which each terminal sulfur atom accommodates two lone pairs of electrons and the anti-bonding levels of the delocalised 7t-system are only partially occupied. 

It wilt be convenient to describe first the binary sulfur nitrides SiN, and then the related cationic and anionic species, S,Ny". The. sulfur imides and other cyclic S-N compounds will then be di.scussed and this will be followed by sections on S-N-halogen and S-N-0 compounds. Several compounds which feature isolated S<—N, S-N, S=N and S=N bonds have already been mentioned in the section on SF4 e.g. F4S-(-NC5H, F5S-NF2. F2S=NCF3, and F SsN (p. 687). However, many SN compounds do not lend themselves to simple bond diagrams, - and formal oxidation states are often unhelpful or even misleading. 

The carefully controlled thermolysis of solutions of [S4N5] in boiling acetonitrile generates [SsNs]" and, subsequently, the [S4N] anion. The reaction of [S4N5]" with bromine or iodine produces pentasulfur hexanitride, SsNs, whereas oxidation with chlorine yields [S4N5]C1. 

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