Vanadium complexes hexacarbonyl

Mono- and diphenylphosphine also give the corresponding dimeric complexes with vanadium hexacarbonyl. 

Vanadium hexacarbonyl reacts with various arenes yielding complexes [(arene)V(CO)4]" [V(CO)6] from which hexafluorphosphate or tetra-phenylborate salts can be prepared by metathesis reaction (50, 52). The cation [(CeHe)V(CO)4] reacts with NaBH4 to give the neutral cyclo-hexadienyl compound (CeH7)V(CO)4 (51). Similar results are observed with methyl-substituted derivatives. The addition of the hydride ion was shown to occur preferentially at unsubstituted positions of the aromatic ring. 

In the case of [Ni(CO)4], [Fe(CO)s], and [M(CO)J where M = Cr,Mo,W, spectra of negative ions were also investigated. For M(CO) -type carbonyls, the following ions were observed M(CO) i, M(CO) 2 M(CO) , M. The parent ions are not stable, because the extra electron must occupy an antibonding molecular orbital. An exception is V(CO)6, a 17e complex, in which the electron occupies a nonbonding orbital. [V(CO)6]- is easily formed in solution it was also found in the mass spectrum of vanadium hexacarbonyl. 

The most general method for the preparation of carbonyl arene complexes is substitution of CO groups in metal carbonyls. Vanadium hexacarbonyl undergoes disproportionation in the presence of arenes ... 

C H, 1,3-Cydopentadiene, l-methyl-, chromium, molybdenum, and tungsten hexacarbonyl complexes, 28 148 preparation and purification of, 27 52 titanium complex, 27 52 vanadium complex, 27 54... 

For example, in Ni(CO) nickel metal having 28 electrons coordinates four CO molecules to achieve a total of 36 electrons, the configuration of the inert gas krypton. Nearly every metal forming a carbonyl obeys the 18-electron rule. An exception is vanadium, forming a hexacarbonyl in which the number of electrons is 35. This carbonyl, which has a paramagnetism equivalent to one unpaired electron, however, readily adds one electron to form a closed valence shell complex containing the V(CO)(, anion. 

The neutral complexes of chromium, molybdenum, tungsten, and vanadium are six-coordinate and the CO molecules are arranged about the metal in an octahedral configuration as shown in stmcture (3). Vanadium carbonyl possesses an unpaired electron and would be expected to form a metal—metal bond. Steric hindrance may prevent dimerization. The other hexacarbonyls are diamagnetic. 

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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