Pentacarbonylpyrazinetungsten(0), (CO)(5)W(pyz), is not stable in solution in polar solvents such as acetone or dichloromethane and undergoes conversion to a bimetallic complex, (CO)(5)W(pyz)W(CO)(5) plus free pyrazine. These three species exist at equilibrium. Using the quantitative H-1 NMR spectroscopy, the equilibrium constant could be determined to be K-eq = (5.9 +/- 0.8) x 10(-2) at 25 degrees C. Introducing a second pyrazine ligand into the molecule does not stabilize the complex, as CiS-W(CO)(4)(pyz)(2) was found to be less stable than W(CO)(5)(pyz) and, therefore, could not be isolated. However, introducing trimethylphosphite as a donor ligand into the complex leads to the stabilization of the carbonyl-pyrazine-metal(0) complexes, as shown by the synthesis Of CiS-W(CO)(4)[P(OCH3)(3)](pyz). This complex could be isolated from the reaction of the photogenerated W(CO)(4)[P(OCH3)(3)](tetrahydrofuran) with trimethylphosphite upon mixing for 2 h at 10 degrees C in tetrahydrofuran and characterized by elemental analysis, IR, MS, H-1, C-13 and P-31 NMR spectroscopy. (c) 2006 Elsevier B.V. All rights reserved.