The novel bulky Schiff base chelate ligand [(4,5-diisopropyl-1H-pyrrole-2-yl)methylene]-4-(tert-butyl)aniline ((HL)-H-iPr2) bearing two isopropyl groups close to the pyrrole nitrogen atom reacts with MoCl2(dme)O-2 (dme = 1,2-dimethoxyethane) to give the sterically congested complex Mo(VI)(L-iPr2)O-2(2) ((iPr2)1; OC-6-4-4 configuration). In spite of the increased steric shielding of the [MoO2] unit (iPr2)1 is active in oxygen-atom transfer to PMe3 and PPh3 to give OPMe3 and OPPh3, respectively. Because of the increased steric bulk of the chelate ligand, formation of dinuclear complexes [Mo-ViPr2)L)O-2](2)(mu-O) ((iPr2)3) by comportionation is effectively prevented in contrast to the highly favored formation of [(MoH2)-H-v)L)(2)O](2)(mu-O) (H(2)3) with the less bulky ligand (H-2)HL. Instead, the smaller PMe3 ligand coordinates to the resulting pentacoordinate intermediate Mo-IViPr2)L)O-2 ((iPr2)5), giving the hexacoordinate complex Mo((IViPr2))L)O-2(PMe3) ((iPr2))(2)) with OC-6-3-3 configuration. The larger potential ligands PPh3 and OPPh3 are only able to weakly coordinate to (iPr2))5, giving labile and sensitive Mo((IViPr2))L)O-2(L) complexes ((iPr2))(6), L = PPh3; (iPr2))7, L = OPPh3). Traces of water and dioxygen in solutions of (iPr2))6/(iPr2))7 yield the di(mu-oxido) complex [Mo-V((iPr2))L)O](2)(mu-O)(2) ((iPr2))(4)) with reduced steric congestion due to dissociation of the bulky chelate ligands. According to electron paramagnetic resonance studies, the much more strongly bound small PMe3 ligand in (iPr2))2 can be slowly liberated by one-electron oxidation to Mo-V, with Ag+ leaving a free coordination site at Mo-V. Hence, essentially pentacoordinate Mo-IV and Mo-V complexes are accessible as a result of the increased steric bulk.