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The Se4I4*+ cation has been prepared by the reaction of Se42+ and iodine in SO2 solution; the 77Se n.m.r. spectrum is consistent with an I2Se+SeSeSe+l2 structure for the cation, and the equilibrium of Se4I42+ with lesser amounts of Se13+ and Se6I2*+.Recently the salts of Se214(Sb2F&,l (Se6I);n(MF6) (M = As, Sb),2 and Se612(A~F6)2.2S02 containing stable selenium-iodine cations have been prepared and characterised in the solid state by X-ray crystallography. We report below the reaction of Se4(A~F6)2 with elemental iodine to form the novel Se4142+ cation in liquid sulphur dioxide, the first selenium iodide to be characterised in solution by 77Se n.m.r. spectroscopy. The spectrum is consistent with an 12Se+SeSeSe+12 structure for the cation [cf. 12Se+Se+12].1 The 77Se n.m.r. spectrum also shows that Se4142+ is in equilibrium with lesser amounts of Se13+ and Se6122+, both compounds previously identified in the solid state. '-3.4 This work shows that although neutral selenium iodides are unknowns selenium-iodine cations, like their sulphur-iodine counterparts , 6 3 7 have an unexpectedly extensive and novel chemistry. In addition, we illustrate the utility of 77Se n.m.r. spectroscopy in investigating the little known chemistry of selenium homopolyatomic cations. Some examples of the chemistry of these cations include their reactions with perflu~ro-olefins,~~~ w(c0)6,10 and CO.11

Ab initio multi-reference configuration interaction (MRD CI) calculations were carried out for the potential energy curves of the first 17 electronic states of the CsH molecule up to large bond distances (20 bohr). The aS+ states were also calculated by means of relativistic all-electron SCF and CI using the spin-free no-pair operator with external field projectors. For the low-lying states, the spectroscopic parameters were determined. Dipole moments as well as the transition dipole moments: # ( X ' S +~ A ~ Z+), #(X 1 Z + B ~ Z+), # ( A ~ S + ~ B ~ S+), were also calculated. Non-relativistic and relativistic results are compared. An analysis of the interactions in the 1,3S+ states is also proposed.