M(CO)4 (M = Mo or W) complexes of 2,6-bis(methylthiomethyl)pyridine (L1) and 2,6-bis(p-tolylthiomethyl)pyridine (L2): a dynamic NMR study

The hybrid S/N/S donor ligands 2,6-bis(methylthiomethyl)pyridine (L 1 ) and 2,6-bis( p -tolylthiomethyl)pyridine (L 2 ) react with the [M(CO) 5 (THF)] (M = Mo or W) compounds to form complexes of general formula [M(CO) 4 L] (M = Mo, L = L 2 ; M = W, L = L 1 or L 2 ), where both L 1 and L 2 act in a S/N bidentate chelate fashion. In solution, these complexes undergo three fluxional processes, viz. inversion at the coordinated S atom, S 1 –S 2 switching, and combined inversion and S 1 –S 2 switching, leading to an interconversion of the four possible permutational isomers. Energy barriers for all three processes have been evaluated by standard one-dimensional band-shape analysis techniques. The mechanism of the S 1 –S 2 switch is discussed.