The reaction of R2P–P(SiMe3)Li (R = tBu, iPr) with the diimido molybdenum complex [(ArN)2MoCl2·dme] (Ar = 2,6-iPr2C6H3; dme = 1,2-dimethoxyethane) yielded the side-on-coordinated phosphanylphosphinidene anionic complexes [(2,6-iPr2C6H3N)2Mo(Cl)(η2-P=PR2)]– (7Mo, 8Mo). The thermal decomposition of [(2,6-iPr2C6H3N)2M(Cl)(η2-P=PR2)]– [M = Mo (7Mo), W (8W)] to [(2,6-iPr2C6H3N)2M(Cl)(1,2-η-tBu2P=P–PtBu2)] [M = Mo (3Mo), W (3W)] was investigated by 31P{1H} NMR spectroscopy. The reactions of [(ArN)2MCl2·dme] (Ar = 2,6-iPr2C6H3; M = Mo, W) with tBu2P–P(Li)–PtBu2 gave molybdenum and tungsten complexes [(2,6-iPr2C6H3N)2M(Cl)(1,2-η-tBu2P=P–PtBu2)] (3Mo, 3W) bearing an unusual triphosphorus ligand. The solid-state structures of [(2,6-iPr2C6H3N)2Mo(Cl)(η2-P=PtBu2)]– (7Mo), [(2,6-iPr2C6H3N)2Mo(Cl)(η2-P=PiPr2)]– (8Mo), [(2,6-iPr2C6H3N)2Mo(Cl)(1,2-η-tBu2P=P–PtBu2)] (3Mo) and [(2,6-iPr2C6H3N)2W(Cl)(1,2-η-tBu2P=P–PtBu2)] (3W) were established by single-crystal X-ray diffraction analysis.