Structure of Niobium and Tantalum carbonyls

$\ce{V(CO)6}$ is well known to be an unusual 17 valence electron, paramagnetic octahedral complex. 


Do the corresponding tantalum and niobium species form simple dimers or more complicated structures ? 




From the abstract of *J. Chem. Theory Comput.*, **2011**, 
*7*, 2112-2125 ([DOI](http://dx.doi.org/10.1021/ct2003513)) is seems that niobium forms a dinuclear $\ce{Nb2(CO)12}$ complex, in which two $\ce{Nb(CO)6}$ units are connected by a 340 pm $\ce{Nb-Nb}$ bond.


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