A Kinetic Study of the Ring-Opening Process in Tungsten Carbonyl Complexes Containing Hemilabile Metallodithiolate Ligands

The synthesis of the metallodithiolate derivative of tungsten pentacarbonyl from the reaction of photogenerated
W(CO)5THF and Ni-1* ((1,5-bis(2-mercapto-2-methylpropane)-1,5-diazacyclooctanato)nickel(II)) is described, along
with its crystal structure. In N,N-dimethylformamide solution, the pentacarbonyl exists in equilibrium with its
tetracarbonyl analogue and carbon monoxide. The pentacarbonyl complex stereoselectively loses cis carbonyl
ligands, as is apparent from 13CO-labeling studies, where the thus-formed tetracarbonyl tungsten complex resulting
from chelate ring-closure is preferentially 13CO-labeled among the two mutually trans CO groups. The kinetics of
the addition of CO to the tetracarbonyl to afford the metal pentacarbonyl were monitored by means of in situ
infrared spectroscopy in the νCO region at CO pressures between 28 and 97 bar and temperatures over the range
45−60 °C. Under these conditions, there was no evidence for W−S bond cleavage in the pentacarbonyl complex
with concomitant formation of W(CO)6. These studies reveal that the tetracarbonyl complex and CO are only slightly
unstable with respect to the formation of the pentacarbonyl complex, with an equilibrium constant at 50 °C of about
2.8 M-1 or ΔG° = −1.4 kJ/mol. The activation parameters determined for the ring-opening process (ΔH⧧ = 89.1
kJ/mol and ΔS⧧ = −37.2 J/mol·K) suggest a solvent-assisted concerted ring-opening mechanism.