The abstraction of the Lewis acid from [W(CO)5(PH2BH2⋅NMe3)] (1) by an excess of P(OMe3)3 leads to the quantitative formation of the first Lewis base stabilized monomeric parent compound of phosphanylborane [H2PBH2⋅NMe3] 2. Density functional theory (DFT) calculations have shown a low energetic difference between the crystallographically determined antiperiplanar arrangement of the lone pair and the trimethylamine group relative to the PB core and the synperiplanar conformation. Subsequent reactions with the main-group Lewis acid BH3 as well as with an [Fe(CO)4] unit as a transition-metal Lewis acid led to the formation of [(BH3)PH2BH2⋅NMe3] (3), containing a central H3BPH2BH2 unit, and [Fe(CO)4(PH2BH2⋅NMe3)] (4), respectively. In oxidation processes with O2, Me3NO, elemental sulfur, and selenium, the boranylphosphine chalcogenides [H2P(Q)BH2⋅NMe3] (Q = S 5 b; Se 5 c) as well as the novel boranyl phosphonic acid [(HO)2P(O)BH2⋅NMe3] (6 a) are formed. All products have been characterized by spectroscopic as well as by single-crystal X-ray structure analysis.