Why is V (CO)6 stable ?
UZS...@ibm.rhrz.uni-bonn.de
Vanadium hexacarbonyl is a quite stable complex although it has an odd
electron number. Why is it so stable ? Other elements like manganese
avoid making such complexes and build Mn2CO10 instead.
Martin
Peter Gans
>From: UZS...@ibm.rhrz.uni-bonn.de
>Subject: Why is V (CO)6 stable ?
>Date: Fri, 04 Nov 94 22:15:01 MEZ
>Keywords: Why is V(CO)6 stable ?
>Vanadium hexacarbonyl is a quite stable complex although it has an odd
>electron number. Why is it so stable ? Other elements like manganese
>avoid making such complexes and build Mn2CO10 instead.
This question arises from the application of the 18-electron rule. On the face
of it V(CO)6 and Mn(CO)5 appear to be isoelectronic in so far as the metal has
a share in 17 electrons in both species. With Mn the 18 electron structure is
achieved by forming an Mn-Mn bond which gives each Mn a share in another
electron. Why does not the same happen with V(CO)6?
The answer lies in the relative stabilities of the components in the
equilibrium 2 V(CO)6 = (CO)6V-V(CO)6
From the fact that dimerization does not take place we must conclude that the
free energy change is large and positive. Certainly the entropy change is
unfavourable (2 particles going to one). The rest is speculation. Perhaps the
enthalpy change is also unfavourable: because of the 7-coordinate nature of V
in the dimer; perhaps V is too small to accomodate a coordination number of 7;
perhaps the pi-bonding is less efficient in the dimer. The nearest analogy I
can come up with is the reluctance of chlorine dioxide to dimerize.
Both [V(CO)6]- and [Mn(CO)5]- are stable 18-electron species.
Peter Gans
School of Chemistry,
The University of Leeds, Tel +44 532 336409
Leeds LS2 9JT, Fax +44 532 336565
England. WWW http://chem.leeds.ac.uk:80/chemistry.html
/people/gans.html
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