Tetranuclear platinum phosphido complexes with different structures

The addition of [NBU4]Br or [NBU4][BH4] to solutions of [Pt-4(mu-PPh2)(4)(C6F5)(4)(CO)(2)] (3) yields the complexes [NBU4](2)[Pt-4(mu-PPh2)(4)(u-X)(2)(C6F5)(4)] (X = Br, 4; H, 5) in which the two CO groups have been replaced by two anionic, bridging X ligands. The total valence electron counts are 64 and 60, respectively; thus, complex 4 does not require Pt-Pt bonds, while two metal-metal bonds are present in 5, as their NMR spectra confirm. Also, the NMR spectra indicate a nonsymmetrical "Pt(mu-PPh2)(2)Pt(mu-PPh2)(mu-X)Pt(mu-PPh2)(mu-X)Pt" disposition for 4 and 5. Treatment of 5 with HX (X = Cl, Br) yields the complexes [NBU4](2)[Pt-4(mu-PPh2)(4)(mu-H)(2)(C6F5)(3)X] (X = Cl, 6; Br, 7). These complexes react with [Ag(OClO3)PPh3] with displacement of the halide and formation of [NBU4][Pt-4(mu-PPh2)(4)(u-H)(2)(C6F5)(3)PPh3] (8). Complexes 6-8 maintain the same basic skeleton as 5, with two Pt-Pt bonds. Complex 4 is, however, an isomer of the symmetric [NBU4](2)[{(C6F5)(2)Pt(mu-PPh2)(2)Pt(mu-Br)}(2)] (9), which has been prepared by a metathetical process from the well-known [NBU4](2)[{(C6F5)(2)Pt(mu-PPh2)(2)Pt(mu-Cl)}(2)] (1). The comparison of the X-ray structures of 4 and 9 confirms the different disposition of the bridging ligands, and their main structural differences seem to be related to the size of Br- and its position in the skeleton.