Unlike the conventional polymeric boron-phosphide-based (BP) semiconductors, which exhibit limited reactivity, the neutral monomeric BP motif (1) is extremely reactive and nonexistent under ambient conditions. Herein, we depict the ligand-engineering strategies for stabilizing the elusive species 1, initially by employing stereoelectronically tuned donor-based ligands, followed by their successive incorporation into the smallest metallacycles with induced aromaticity, and hence excellent stability. The electron density distribution and chemical bonding of homo- and heterobileptic ligand-stabilized monomers [(L′)BP(L)] (2–7) [L′, L = singlet carbenes], and the corresponding neutral 3-membered metal(II)dihalide complexes [BP(MX2)] (8–9′) and [((L′)BP(L))(MX2)] (10–17) [M = Pd/Pt, X = Cl, Br], are investigated by various quantum chemical methods. The remarkable ligand-switched σ and π aromaticity in the unprecedented mixed d- and p-block planar metallacycles is unambiguously confirmed by NICSzz calculations, ELF, AdNDP, GIMIC, and EDDB analyses.