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The complexes fac-[PtMe3L]CI, where L = 1,4,7-trithiacycIononane ( l ) , 1,4,7,1O-tetrathiacyclododecane (21, and 10-oxa-2,4,7-trithiacyclododecane (3), have been prepared, and the X-ray crystal structure of (1 1, isolated as a dihydrate, has been obtained; variable-temperature 1H and 13C-{lH} n.m.r. studies of (2) have revealed a novel ligand commutation involving exchange between metal-co-ordinated and unco-ordinated sulphur atoms of the macrocycle.

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The monomer-dimer equilibrium of nitrosobenzene in CDCl, solution was studied in the temperature range -50 to 30 "C using "C, 15N and "0 NMR spectroscopy. Decreasing temperature favours the dimeric azodioxy species, and below ca 10 "C both cis and trans dimers were detected in addition to the monomer. The relative abundance of the trans dimer is low (<6%) at all temperatures and this is the first report of its existence in solution. The monomer and dimer species were characterized by their I3C and I5N shifts, and exchange between them was measured quantitatively by I3C 2D-EXSY spectra. Exchange occurred exclusively between the cis-dirner and monomer and between the trans-dimer and monomer, activation energies, AG'(298.15 K) being 65.8k 0.1 and 70.0k0.3 kJ mol-', respectively.

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