Ajaya, you need to be more specific about the type of materials you are interested in.  Molecular compounds?  Metal oxides, sulfides or other solid materials?  However, in general, low and even negative oxidation states are obtainable with ligands that can stabilize that charge by pi backhanding.  Carbon monoxide ligands do a very good job at that so you will find compounds of Nb with CO ligands in -1 and -3 states.  http://www.ncbi.nlm.nih.gov/pubmed/11670773

 Also check out John Ellis's papers:

http://pubs.acs.org/doi/abs/10.1021/ic052110i

http://pubs.acs.org/doi/abs/10.1021/om030105l 

I second the suggestion regarding John Ellis' papers.  He has done amazing things with low oxidation state metal compounds.  In addition, even though this is quite different than what you have asked about, I thought I would mention the great work of James Dye in making negative oxidation states of the alkaline metals.  http://rsta.royalsocietypublishing.org/content/roypta/373/2037/20140174.full.pdf