Crystal and Molecular Structures of Dihalotetrakis(pyrophosphito)diplatinum(III) Complexes. Integrative Use of Structural and Vibrational Data To Assess Intermetallic Bonding and the Trans Influence of the Pt(III)-Pt(III) Bond

The structures of [N(C4H9)4]4[Pt2O5H2Br2] (III), K4[Pt2(P2O5H2)4I2] (IV), and K2[N(C4H9)4]2[Pt2(P2O5H2)4I2] (V) show four P-bonded μ-pyrophosphito groups spanning axially halo-disubstituted diplatinum(III) centers. The Pt-Pt bond lengths vary as the halide (X) is X- = I- > Br- > C1-, in accord with their σ-donating strength. In addition, the axial Pt-X bonds are long, reflecting a high trans influence for the Pt(III)-Pt(III) bond. Compound III is monoclinic, space group P21/n, with a = 16.047 (4) Å, b = 16.850 (3) Å, c = 16.331 (3) Å, β = 92.00 (2)°, V = 4413 (2) Å3, and Z = 2; IV is tetragonal, space group I4/m, with a = 9.399 (1) Å, c = 15.777 (2) Å, V = 1393.7 (5) Å3, and Z = 2; V is monoclinic, space group P21/n, with a = 9.749 (3) Å, b = 20.937 (8) Å, c = 15.242 (6) Å, β = 91.67 (3)°, V= 3110 (2) Å3, and Z = 2. The trans influence for the Pt(III)-Pt(III) bond is estimated from vibrational spectroscopic data and found to be stronger than that for halides but weaker than that for methides, phenides, or hydrides.