Please do not adjust margins Due to the side-on coordination of the phosphinoboryl ligand, the B1-Pt1-P1 angle was acute, measuring 52.55(7)°, while the other angles around the Pt1 atom were obtuse, ranging from 93.75(2)° to 112.10(7)°. The Pt1-B1 bond distance of 2.020 Å was very close to analogous distances reported for aminoboryl platinum complexes. The Pt1-P1 bond (2.286 Å) was slightly shorter than the distance between Pt1 and the P2 atom of the PCy3 ligand (2.323 Å). Moreover, the B-P bond within the phosphinoboryl ligand (1.921(3) Å) was somewhat shorter than the expected length for a single covalent bond (1.96 Å). The B1 and N1 atoms exhibited planar trigonal geometries, with the sums of the angles around these atoms close to 360°. This is consistent with the significantly shortened B1-N1 bond distance (1.385(3) Å) and suggests π-donation from the N1 atom to the B1 atom. Furthermore, the Pt1-Br bond distance of 2.608 Å was notably longer than the typical Pt-Br bond length (2.37 Å), which can be attributed to the strong trans effect of the phosphinoboryl ligand. The electronic structure of 2a was investigated through theoretical methods. According to our findings, the HOMO of 2a was delocalized mainly at the Pt1-B1-P1 triangle in a bonding manner, with some contribution from the lone electron pair on the Br1 atom (of nonbonding character) (Fig. ). The LUMO was also based on the Pt1-B1-P1 region (in a bonding manner) but with antibonding contributions between the Pt1-B1-P1 triangle and neighboring atoms (Fig. ). According to natural bond orbital analysis, the largest positive charges were located on the P2 (+1.08), P1 (+0.57), and B1 (+0.50) atoms, whereas the platinum center carried only a slight positive charge (+0.13). The largest negative charges were found on the N1 (-0.81) and Br1 (-0.78) atoms. The phosphinoboryl ligand was stabilized by π-donation from the N1 atom to a formally empty, unhybridized p orbital of boron, consistent with the planar trigonal geometry around these atoms. The lone pair on the P1 atom interacted not only with the metal center, forming a dative bond but also, to a lesser extent, with the antibonding π*(B1-N1) orbital, shortening the B1-P1 bond. The calculated 57% contribution of Pt to the σ(Pt1-B1) bond confirmed the covalent character of this bond. The ratio of boron p-to s-orbital contributions in the σ(Pt1-B1) bond was 1:1.82, indicating the strong σ-donor properties of the phosphinoboryl ligand. These properties led to the strong trans effect of this ligand, which manifested in a significant elongation of the Pt1-Br1 bond. In conclusion, we developed a simple synthetic route for sideon phosphinoboryl complexes. The presence of reactive Pt-B and B-P bonds in these structures makes them promising candidates for applications in small molecule activation and catalysis. Financial support of these studies from Gdańsk University of Technology by the DEC-2/2021/IDUB/V.6/Si grant under the SILICIUM -'Excellence Initiative -Research University' program and Minigrant No. 037741 is gratefully acknowledged. The calculations have been carried out using resources provided by Wroclaw Centre for Networking and Supercomputing () grant No. 435.