Sulfur mononitride is an inorganic compound with the molecular formula SN. It is the sulfur analogue of and isoelectronic to the radical nitric oxide, NO. It was initially detected in 1975, in outer space in giant molecular clouds and later the coma of comets. This spurred further laboratory studies of the compound. Synthetically, it is produced by electric discharge in mixtures of nitrogen and sulfur compounds, or combustion in the gas phase and by photolysis in solution.
Synthesis The NS radical is a highly transient species, with a lifetime on the order of milliseconds, but it can be observed spectroscopically over short periods of time through several methods of generation. NS is too reactive to isolate as a solid or liquid, and has only been prepared as a vapor in low pressure or low-temperature matrices due to its tendency to rapidly oligomerize to more stable, diamagnetic species.
Electric discharge Transmission of electric discharge through a glass tube with quartz windows containing a mixture of nitrogen and sulfur vapor (rigorously free of oxygen) results in the spectrum of emitted light gaining bands consistent with the formation of NS. Passing a mixture of gaseous N2 and S2Cl2 through the side arm of an absorption cell undergoing microwave discharge produces NS. Infrared diode laser spectroscopy taken using this method allowed for derivation of the equilibrium rotational constant, and therefore calculation of the equilibrium bond length as 1.4940 Å. With low pressure microwave discharge of elemental nitrogen and sulfur, followed by low temperature trapping in argon matrices, one obtains a mixture of products including NS, NNS, SNS, and NSS. By adding excess sulfur, SSNS is also produced.
Sulfur- and nitrogen-doped flames Methane was premixed with fuel in the form of either O2, N2O, or air and burned at ambient pressure. The source of nitrogen was introduced by addition of 1–5 mol% NH3 gas and sulfur by 0.01–0.5 mol% H2S or SF6 gas. A steady state concentration of NS within the flame front is observed by laser-induced fluorescence (LIF) spectrum.
Photolysis The NS radical was detected by LIF spectrum as the product of photolysis of tetranitrogen tetrasulfide (N4S4) gas by a 248 nm laser. Aerated solutions of Cr(CH3CN)5(NS)2+ are highly photoactive and prone to rapid decomposition. Deaerated solutions of Cr(CH3CN)5(NS)2+ in acetonitrile are stable as long as they are kept in the dark. Continuous photolysis using 366 nm light is slow, while using a 355 nm pulsed laser results in faster labilization of NS.
Reactivity
Oligomerization Evidence suggests that NS can react with itself to reach N2S2, N4S4, and polymers of the form (NS)x. (NS)x forms from polymerization of cyclo-N2S2.
Trans-NSSN results from direct dimerization of NS.
The decay time associated with these reactions is on the order of 1-3 ms. N3S3 has been observed through photoelectron spectroscopy of vapors of the (SN)x, polymer, but has not yet been characterized further. Attempts to produce N3S3 by oxidation of [PPN][S3N3] were unsuccessful. It is theorized that rapid dimerization to (N3S3)2 will disproportionate irreversibly to N4S4 and N2S2.
Combustion NS does not react with NO or O2 at ambient temperatures. Addition of NO2 induces rapid, first-order decay with a rate constant (295 K) of k = (2.54 ± 0.12) × 10−11 cm3 molecules−1 s−1. This reaction is proposed to proceed through various intermediates, ultimately reaching final products of N2 and SO2.
Metal-thionitrosyl complexes As a ligand, NS acts as a σ-donor and π-acceptor, forming metal-thionitrosyl complexes. Transition-metal thionitrosyl complexes have been prepared by the following procedures:
Sulfur transfer to metal nitrido complexes. For example, reflux of (Ph4P)[OsNCl4] and (Ph4P)NCS yields green-brown solid [Ph4P]2[Os(NS)(NCS)5]. However, "this route is not readily generalised". Reaction of trithiazyltrichloride with transition metal complexes, e.g.: NSCl3 + OsCl3 → [Os(NS)Cl3] Halide abstraction from coordinated thiazyl complexes, e.g.: [(η5−C5H5)Cr(NO)2(NSF)]-[AsF6] + AsF5 → [(η5-C5H5)Cr(NO)2(NS)]-[AsF6]2 Reaction of NS+ salts with transition metal complexes, e.g.: NS+SbF6− + [M(CO)5Br] → [M(CO)5(NS)]2+, M=Mn, Re NS+AsF6− + [(η5-C5H5)Fe(CO)2(SO2)]+ → [(η5-C5H5)Fe(CO)2(NS)][AsF6]2 The NS+ salts themselves can be made from thiazyl chloride and silver hexafluorophosphate. Historically, tetrasulfur tetranitride was believed to react with metal halides or nitrides to give thionitrosyl complexes. However, the reaction only proceeds in protonic solvents, and in fact only breaks one bond to form the bidentate -S-N-S-NH- ligand. From X-ray crystallography of many of such metal-thionitrosyl complexes, one can observe that the M-N-S bond angle is nearly linear, suggesting sp hybridization about N. Short M-N distances and long N-S distances reflect the resonance structure of M=N=S having greater contribution than M-N≡S. Typical v(NS) IR stretching frequencies are approximately 1065 cm−1 for low-valent transition metal complexes and around 1390 cm−1 in the high valent cases, whereas the free gas-phase radical exhibits a 1204 cm−1 signal.
NS transfer When a spin-trapping agent, such as Fe(S2CNEt2)2 is present during the photolysis of Cr(CH3CN)5(NS)2+, new S=1/2 EPR bands are observed, attributed to the formation of Fe(S2CNEt2)2(NS), and the signal from Cr(CH3CN)5(NS)2+ disappears. This suggests that the NS radical has transferred from the chromium complex to the iron complex. An example of an NS in situ transfer is the following reaction, which needs light to occur:
(Cr(CH3CN)5(NS)2+ + Fe(S2CNEt2) + CH3CN → Cr(CH3CN)2+6 + Fe(S2CNEt2)2(NS) This was particularly significant as it was the first controlled and well-characterized reactivity of NS in solution. Further, it showed the potential for similar reactivity in known reactions with NO, such as use of this iron dithiocarbamate complex.
Bonding
The valence electrons of this compound match those of nitric oxide. Sulfur mononitride can be described as some average of a set of resonance structures. The singly bonded structure (first resonance structure shown) has little contribution. The formal bond order is considered to be 2.5.
Versus NO The decreasing electronegativity with increasingly heavy chalcogenides leads to a reversal of the dipole. In NO, oxygen is the more electronegative element. In NS, nitrogen is more electronegative. The NS radical is significantly more unstable and prone to catenation than NO.
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