Transition metal boryl complexes have been extensively studied computationally and experimentally due to the strong σ-donors of Lewis acidic boryl ligands with minimal π-backbonding to the metal center . Many groundbreaking discoveries have been made since the first mention of boryls in the 1960s . These complexes are well established and have been used as key intermediates in numerous reactions such as diboration , hydroboration , the activation of CO2 and CS2 , and C-H borylation . Several methods have been used for the preparation of mono-, bis-, and tris-transition-metal boryl complexes. A method widely used to obtain mono-and bis-boryl complexes is the oxidative addition of diboron (B-B) or borane (B-X, X = H or halogen) to low-valent metal centers to provide bis(boryl) and boryl complexes, respectively. Braunschweig reported iron, ruthenium, and platinum complexes bearing ferrocenyl(bromo)boryl ligands as well as cationic platinum(II) boryl complexes demonstrating the high trans influence of the boryl group and applied trans-[(Cy3P)2Pt(Br){B(Mes)Br}] (Cy = cyclohexyl, Mes = mesityl) in the synthesis of base-free platinum(II) borylene complexes . Furthermore, the trisboryl complexes [(Me3P)3Co(Bcat)3] 42 and [xant( have also been investigated. Notably, complexes bearing PBP polydentate borane and boryl ligands have found application in hydrogen activation , as catalysts in the dehydrogenation of dimethylamine and the hydrosilylation of aldehydes , and as pre-catalysts in ethylene polymerization (Chart 1A). Interestingly, the cobalt(I) diphosphinoboryl complex underwent reversible double H2 activation . While platinum complexes bearing directly bonded low-valent P-B ligands have been previously described by Bourissou with phosphinoborane (Chart 1B) and by Drover and our group with diphosphinoboranes, no transition metal boryl complexes with directly bonded boron and phosphorus centers have been reported to the best of our knowledge. In this work, we synthesized and characterized the first platinum(II) complexes with a side-on bonded phosphinoboryl ligand featuring a direct B-P bond (Chart 1C). Recently, we used bromo(phosphino)boranes as substrates to synthesize phosphinoborinium and phosphinoborenium cations via bromide anion abstraction. The presence of reactive B-Br and B-P bonds in these boranes encouraged us to explore them as precursors for boryl ligands. Furthermore, we supposed that the introduction of a donor phosphanyl group to the boron atom will facilitate the side-on coordination of the boryl ligand to the transition metal center. First, we tested the reactivity of iPr2N(Br)BPtBu2 (a) with [(C2H4)Pt(PPh3)2] (1) in dichloromethane (Scheme 1). The reaction was initiated at -50 °C, after which the reaction mixture was gradually warmed to room temperature and stirred for 72 h. The reaction progress was monitored by The desired phosphinoboryl Pt(II) complex (1a) was isolated as yellow crystals by diffusion crystallization from a toluene/pentane system at room temperature. The 31 P-NMR spectrum of 1a displayed two doublets at 42.5 and -67.2 ppm with a 2 JPP coupling constant of 265 Hz, which were respectively attributed to the phosphine ligand and tBu2P fragment of the phosphinoboryl ligand. The large 2 JPP coupling constant indicated that the phosphine ligand and the phosphorus atom of the phosphinoboryl group were positioned trans to each other. Both resonances exhibit platinum satellites, with coupling constants of 3314 Hz (PPh3) and 1168 Hz (iPr2NB-PtBu2), confirming the coordination of the phosphorus atoms to the platinum(II) center. Notably, the 31 P NMR resonance of the phosphinoboryl ligand is significantly upfield shifted relative to the phosphorus resonance of the parent borane (-11.2 ppm). The 11 B NMR spectrum of 1a showed a single broad resonance at 40.4 ppm, similar to the corresponding resonance of the borane (39.5 ppm), suggesting the presence of a tricoordinate boron atom. Furthermore, the oxidative addition of the B-Br bond of the parent bromo(phosphino)borane to the Pt(0) center was unambiguously confirmed by the X-ray diffraction (XRD) analysis of the crystals of 1a (for the molecular structure of this complex, see Fig. ). Despite the successful isolation of the target complex 1a, NMR analysis of the reaction mixture indicated the formation of several unidentified byproducts, explaining the low isolation yield of 1a (18%). This finding encouraged us to search for more effective alternative metal precursors for side-on phosphinoboryl complexes. We turned our attention to Pt(PCy3)2 (2), which had been previously used as a substrate to synthesize numerous platinum boryl complexes. Unlike the reaction involving 1, precursor 2 cleanly reacted with iPr2N(Br)BPtBu2 (a) in toluene. Based on 31 P NMR spectroscopy, the reaction mixture contained only the target side-on phosphinoboryl complex 2a and free PCy3 as products (Fig. ). This observation aligned with the proposed stoichiometry for the reaction of 2 with a (Scheme 1). The NMR spectroscopic features of 2a were similar to those observed for 1a, displaying a broad singlet in the B NMR spectrum at 39.5 ppm and two coupled doublets with platinum satellites in the 31 P spectrum: δ = 40.3 ppm (d, 2 JPP= 239 Hz, 1 JPPt = 3148 Hz, PCy3); δ = -75.9 ppm (bd, 2 JPP= 239 Hz, 1 JPPt = 1080 Hz, tBu2P). Evaporating the solvent, dissolving the residue in petroleum ether, and then storing the solution at -20 °C provided 2a as yellow crystals with a 61% yield. The formation of 2a was further confirmed by single-crystal XRD (Fig. ).