4-2 oxidation state hexacarbonyl

The O oxidation state is known in vanadium hexacarbonyl. V(CO)(,. a blue-green, sublimable solid. In the molecule VfCO), if each CO molecule is assumed to donate two electrons to the vanadium atom, the latter is still one electron short of the next noble gas configuration (krypton) the compound is therefore paramagnetic, and is easily reduced to form [VfCO, )]. giving it the... 

In equation 1, the Grignard reagent, C H MgBr, plays a dual role as reducing agent and the source of the arene compound (see Grignard reaction). The Cr(CO)g is recovered from an apparent phenyl chromium intermediate by the addition of water (19,20). Other routes to chromium hexacarbonyl are possible, and an excellent summary of chromium carbonyl and derivatives can be found in reference 2. The only access to the less stable Cr(—II) and Cr(—I) oxidation states is by reduction of Cr(CO)g. 

Chromium forms a white solid, hexacarbonyl, Cr(CO)6, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [Cr(CO)5I] . 

The prototypically zero oxidation state complexes of the group are the binary hexacarbonyls M(CO)6. Early studies of the electrochemistry of these 18-electron closed-shell systems in nonaqueous electrolytes has perhaps been seminal in understanding the electron-transfer reactions of more substituted systems and of metal carbonyls in general. 

Additional evidence that reduction is not the role of R AlCla.. in catalyst formation is provided by the observation that the complexes [Bu4N] [Mo(CO)5X] and [R4N] [Mo(CO)5COR L in which the molybdenum is in a low oxidation state, require an organoaluminum reagent for catalytic activity (44, 45). In these examples, the function of the organoaluminum is most likely the removal of CO ligands to make available sites for olefin coordination. Molybdenum hexacarbonyl alone is reported to be a disproportionation catalyst in this case expulsion of the CO groups is attained thermally at 98 °C (46). 

W). Interesting cases of intermediate oxidation states are the hexacarbonyl metalates(—11) of the group 4 metals (Ti, Zr, Hf) of formula [M(CO)6] . Some relevant examples of anionic, neutral, and cationic binary metal carbonyls are listed in Table 1. 

Cr(CO)sI is isoelectronic with V(CO)6 and like the latter exhibits the expected paramagnetism for a species containing one unpaired electron. Chromium pentacarbonyl iodide appears to be more stable to oxidation but less stable to thermal decomposition than vanadium hexacarbonyl. This higher oxidative stability of Cr(CO)5l as compared with that of V(CO)j may be due to the higher formal oxidation state of the central metal atom of Cr(CO)5l. 

In 1964, Fischer prepared and characterized unambiguously the first metal carbene complex 3 obtained by nucleophilic attack of phenyl lithium at tungsten hexacarbonyl followed by 0-alkylation. This was followed by Schrock s synthesis of a high oxidation state metal alkylidene complex 4 obtained by a-hydrogen abstraction from tris(neopentyl) tantalumfv) dichloride (Scheme 1.1)/ ... 

It can be seen also that the 18-electron rule will tend to break down when the first-row transition metals are in a high oxidation state. There may also be steric limitations to the formation of 18-electron complexes in high oxidation states - consider for example the hypothetical cation [Cr(CO)9] . Steric limitations may also explain why vanadium forms the monomeric, paramagnetic and 17-electron hexacarbonyl, V(CO)6, rather than the dimeric complex [V(CO)6]2 which would be diamagnetic and would obey the 18-electron rule. 

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