Electronic structure and chain-length effects in diplatimun polyynediyl complexes trans, trans-[(X) (R3P)2Pt(C=C)nPt(PR3)2(X)]:: A computational investigation

Structure and bonding in the title complexes are studied using model compounds trans, trans-[(C6H5)(H3P)(2)Pt(C=C)(n)Pt(PH3)(2)(C6H5)] (PtCxPt; x = 2n=4-26) at the B3LYP/LACVP* level of density functional theory. Conformations in which the platinum square planes are parallel are very slightly more stable than those in which they are perpendicular (DeltaE = 0.12 kcal mol(-1) for PtC8Pt). As the carbon-chain length increases, progressively longer CdropC triple bonds and shorter dropC-Cdrop single bonds are found. Whereas the triple bonds in HCxH become longer (and the single bonds shorter) as the interior of the chain is approached, the PtCdropC triple bonds in PtCxPt are longer than the neighboring triple bond. Also, the Pt-C bonds are shorter at longer chain lengths, but not the H-C bonds. Accordingly, natural bond orbital charge distributions show that the platinum atoms become more positively charged, and the carbon chain more negatively charged, as the chain is lengthened. Furthermore, the negative charge is localized at the two terminal CdropC atoms, elongating this triple bond. Charge decomposition analyses show no significant d-pi* backbonding. The HOMOs of PtCxPt can be viewed as antibonding combinations of the highest occupied pi orbital of the sp-carbon chain and filled inplane platinum d orbitals. The platinum character is roughly proportional to the Pt/C-x/Pt composition (e.g., x=4, 31%; x = 20, 6 %). The HOMO and LUMO energies monotonically decrease with chain length, the latter somewhat more rapidly so that the HOMO-LUMO gap also decreases. In contrast, the HOMO energies of HCxH increase with chain length; the origin of this dichotomy is analyzed. The electronic spectra of PtC4Pt to PtC10Pt are simulated. These consist of two pi-pi* bands that redshift with increasing chain length and are closely paralleled by real systems. A finite HOMO-LUMO gap is predicted for PtCinfinityPt. The structures of PtCxPt are not strictly linear (average bond angles 179.7degrees-178.8degrees), and the carbon chains give low-frequency fundamental vibrations (x=4, 146 cm(-1); x=26, 4 cm(-1)). When the bond angles in PtC12Pt are constrained to 174degrees in a bow conformation, similar to a crystal structure, the energy increase is only 2 kcal mol(-1). The above conclusions should extrapolate to (CdropC)(n) systems with other metal endgroups.