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) 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.