Comparison of the Properties of SnCl(3)(-) and SnBr(3)(-) Complexes of Platinum(II)

The complexes M(3)[Pt(SnX(3))(5)] (M = Bu(4)N(+), PhCH(2)PPh(3)(+); X = Cl, Br), cis-M(2)[PtX(2)(SnX(3))(2)] (M = Bu(4)N(+), PhCH(2)PPh(3)(+), CH(3)PPh(3)(+), Pr(4)N(+); X = Cl, Br), and [PhCH(2)PPh(3)](2)[PtBr(3)(SnBr(3))] have been prepared and characterized by (119)Sn and (195)Pt NMR, far-infrared, and electronic absorption and emission spectroscopies. In acetone solutions the [Pt(SnX(3))(5)](3)(-) ions retain their trigonal bipyramidal structures but are stereochemically nonrigid as evidenced by (119)Sn and (195)Pt NMR spectroscopy. For [Pt(SnCl(3))(5)](3)(-) spin correlation is preserved between 183 and 363 K establishing that the nonrigidity is due to intramolecular tin site exchange, probably via Berry pseudorotation. Whereas, [Pt(SnCl(3))(5)](3)(-) does not undergo loss of SnCl(3)(-) or SnCl(2) to form either [Pt(SnCl(3))(4)](2)(-) or [PtCl(2)(SnCl(3))(2)](2)(-), [Pt(SnBr(3))(5)](3)(-) is not stable in acetone solution in the absence of excess SnBr(2) and forms [PtBr(2)(SnBr(3))(2)](2)(-) and [PtBr(3)(SnBr(3))](2)(-) by loss of SnBr(2). Similarly, [PtCl(2)(SnCl(3))(2)](2)(-) is stable in acetone at ambient temperatures but disproportionates at elevated temperatures and [PtBr(2)(SnBr(3))(2)](2)(-) loses SnBr(2) in acetone to form [PtBr(3)(SnBr(3))](2)(-). The crystal structures of methyltriphenylphosphonium cis-dibromobis(tribromostannyl)platinate(II) and benzyltriphenylphosphonium tribromo(tribromostannyl)platinate(II) have been determined. Both compounds crystallize in the triclinic space group P&onemacr; in unit cells with a = 12.293(16) Å, b = 12.868(6) Å, c = 25.047(8) Å, alpha = 96.11(3) degrees, beta = 91.06(3) degrees, gamma = 116.53(3) degrees, rho(calc) = 2.30 g cm(-)(3), Z = 3 and with a = 11.046(7) Å, b = 14.164(9) Å, c = 22.549(10) Å, alpha = 89.44(4) degrees, beta = 83.32(5) degrees, gamma = 68.31(5) degrees, rho(calc) = 1.893 g cm(-)(3), Z = 2, respectively. Least-squares refinements converged at R = 0.057 and 0.099 for 4048 and 4666 independent observed reflections with I/sigma(I) > 3.0 and I/sigma(I) > 2.0, respectively. For the former, the asymmetric unit contains 1.5 cis-[PtBr(2)(SnBr(3))(2)](2)(-) ions, 0.5 of which is disordered in such a way as to be pseudocentrosymmetric. This disordering involves a half-occupied PtBr(2) unit appearing on either side of the center. Simultaneously, one bromine from each SnBr(3) ligand changes sides while the other two bromines appear in average positions with very small displacements between their positions. The Pt-Sn distance in [PtBr(3)(SnBr(3))](2)(-) (2.486(3) Å) is slightly shorter than that incis-[PtBr(2)(SnBr(3))(2)](2)(-) (2.4955(3) Å, average), and both are significantly longer than that previously found in cis-[PtCl(2)(SnCl(3))(2)](2)(-) (2.3556 Å, average), which is not consistent with the relative magnitudes of the (1)J((195)Pt-(119)Sn) coupling constants (28 487, 25 720, and 27 627 Hz, respectively). From our electronic absorption and emission studies of the Pt-SnX(3)(-) complexes, we conclude that (a) the low-energy transitions are d-d transitions analogous to those found in [PtX(4)](2)(-) systems, (b) the SnCl(3)(-) ligand is a stronger sigma donor than SnBr(3)(-), (c) the triplet state from which the emission occurs is split by spin-orbit coupling into different spin-orbit states, (d) a forbidden spin-orbit state must lie at or near the bottom of the spin-orbit manifold, (e) the solid state crystal environment perturbs the platinum-tin halide electronic states, and (f) dispersion of the samples in solvents changes this perturbation, which can be rationalized in terms of an in-plane distortion of the square planar platinum coordination sphere.