Spectroscopy of Hydrothermal Reactions 21. Decomposition Reactions and Kinetics of Ni(CO)4, Co(CO)4- and (η5-C5H5)Co(CO)2 in High Temperature Water

The decomposition behavior of aqueous Ni(CO) 4, NaCo(CO) 4, and (η 5 -C 5 H 5 )Co(CO) 2 was studied by infrared spectroscopy using a titanium flow reactor with sapphire windows. A pressure of 275 bar was applied to prevent gas−liquid phase separation. Ni(CO) 4 was investigated in the 50−270 °C temperature range with selected partial pressures of CO in the range of 0−60 bar. Without added CO, the Arrhenius parameters were E a = 10.4 ± 1.3 kcal/mol and ln( A, s -l ) = 10.4 ± 1.9 in the 50−90 °C range where a first-order rate law was followed. Complete decomposition took place at 170 °C within 23 s. The addition of excess CO raised the decomposition temperature. Aqueous Ni(CO) 4 could be detected for at least 5 s at 270 °C with a 60 bar partial pressure of CO. The isoelectronic ion Co(CO) 4 - was found to be more stable than Ni(CO) 4 . Its decomposition was characterized in the 200−240 °C range without added CO. A first-order rate law was followed up to 50% decomposition, yielding E a = 29.9 ± 3.7 kcal/mol and ln( A, s -1 ) = 25.9 ± 3.8. The greater robustness of Co(CO) 4 compared to Ni(CO) 4 is attributable to the strength of the metal−carbon bond. (η 5 -C 5 H 5 )Co(CO) 2 decomposed in the 220−240 °C range to Co(CO) 4 - and a considerable amount of insoluble residue. Decomposition reactions for the three studied metal carbonyls were formulated.