Electronic Structure and Chain-Length Effects in Diplatinum Polyynediyl Complexes trans,trans-[(X)(R3P)2Pt(C≡C)nPt(PR3)2(X)]: A Computational Investigation

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Structure and bonding in the title complexes were studied using model compds. trans,trans-[Ph(H3P)2Pt(C.tplbond.C)n Pt(PH3)2Ph](PtCxPt; x = 2n = 4-26) at the B3LYP/LACVP* level of d. functional theory. Conformations in which the Pt square planes are parallel are very slightly more stable than those in which they are perpendicular (DE = 0.12 kcal mol-1 for PtC8Pt). As the C-chain length increases, progressively longer C.tplbond.C triple bonds and shorter .tplbond.C-C.tplbond. 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 PtC==C 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 Pt atoms become more pos. charged, and the C chain more neg. charged, as the chain is lengthened. Also, the neg. charge is localized at the two terminal C.tplbond.C atoms, elongating this triple bond. Charge decompn. analyses show no significant d-p* backbonding. The HOMOs of PtCxPt can be viewed as antibonding combinations of the highest occupied p orbital of the sp-C chain and filled in-plane Pt d orbitals. The Pt character is roughly proportional to the Pt/Cx/Pt compn. (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 p-p* bands that red shift with increasing chain length and are closely paralleled by real systems. A finite HOMO-LUMO gap is predicted for PtC2Pt. The structures of PtCxPt are not strictly linear (av. bond angles 179.7 Deg-178.8 Deg), and the C 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 174 Deg in a bow conformation, similar to a crystal structure, the energy increase is only 2 kcal mol-1. The above conclusions should extrapolate to (C.tplbond.C)n systems with other metal endgroups.
Original languageEnglish
JournalChemistry: A European Journal
Pages (from-to)6510-6522
Publication statusPublished - 2004
Externally publishedYes

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