The　structural　and　electronic　properties　of　two　series　of　Group　VB　transition　metal　oxide　clusters,　M4On-　and　M4On　(M　=　Nb,　Ta;　n　=　8-11),　are　investigated　using　density　functional　theory　calculations.　Generalized　Koopmans＇　theorem　is　applied　to　predict　the　vertical　detachment　energies　and　simulate　the　photoelectron　spectra.　Large　highest　occupied　molecular　orbital-lowest　unoccupied　molecular　orbital　gaps　are　observed　for　these　two　stoichiometricM4O10　clusters　and　estimated　to　be　3.98　and　4.38　eV　for　M　=　Nb　and　Ta,　respectively.　The　M4O10-/0　(M　=　Nb,　Ta)　clusters　are　polyhedral　cage　structures　with　high　symmetry　(Td　for　the　neutral　and　D2d　for　the　anion)　in　which　each　metal　atom　joints　three　bridging　and　one　terminal　O　atoms.　For　the　Nb　oxide　species,　Nb4O8　-/0　and　Nb4O9-/0　can　be　viewed　as　removing　two　and　one　terminal　O　atoms　from　Nb4O10-/0,　respectively.　The　Ta　species　follow　the　same　rule　to　the　Nb　species,　except　that　the　anionic　Ta4O8-　is　formed　by　removing　one　terminal　and　one　bridging　O　atoms　from　Ta4O10　-.　The　Ta4O9　containing　a　localized　Ta3+　site　can　readily　react　with　O2　to　form　the　Ta4O11　which　can　also　be　viewed　as　replacing　a　terminal　oxygen　atom　in　Ta4O10　by　a　peroxo　O2　unit,　whereas　the　added　oxygen　atom　is　found　to　be　a　bridging　one　in　the　O-rich　clusters　Nb4O11-/0　and　the　anionic　Ta4O11-.　Molecular　orbital　analyses　are　performed　to　analyze　the　chemical　bonding　in　the　tetra-nuclear　metal　oxide　clusters　and　to　elucidate　their　structural　and　electronic　evolution.　©　Springer-Verlag　Berlin　Heidelberg　2014.