Structure of Niobium and Tantalum carbonyls
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.