Electronic Structure and Chain-Length Effects in Diplatinum Polyynediyl Complexes trans,tmns~[(X)(R_3P)_2Pt(C=C)_nPt(PR_3)_2(X)]:A Computational Investigation

摘要

Structure and bonding in the title complexes are studied using model compounds trans,trans-[(C_6H_5)(H_3P)_2-Pt(C=C)_nPt(PH_3)_2(C_6H_5)](PtC.Pt;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(N=(mkcalmol~(-1)for PtC_8Pt).As the carbon-chain length increases,progressively longer C(ident to)C triple bonds and shorter(ident to)C- C(ident to)single bonds are found.Whereas the triple bonds in HC(ident to)H become longer(and the single bonds shorter)as the interior of the chain is approached,the PtC(ident to)C triple bonds in PtC_xPt 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 C(ident to)C atoms,elongating this triple bond.Charge decomposition analyses show no significant d-pi~* backbonding.The HOMOs of PtC_xPt can be viewed as antibonding combinations of the highest occupied it orbital of the sp-carbon chain and filled in-plane platinum d orbitals.The platinum character is roughly proportional to the Pt/CyPt 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 HCJI increase with chain length;the origin of this dichotomy is analyzed.The electronic spectra of PtC_4Pt to PtC_10Pt are simulated.These consist of two ir-it* bands that redshift with increasing chain length and are closely paralleled by real systems.A finite HOMO-LUMO gap is predicted for PtCLPt.The structures of PtC_xPt are not strictly linear(average bond angles 179.7deg-178.8deg ),and the carbon chains give low-frequency fundamental vibrations(x = 4,146cm~(-1);x = 26,4cm~(-1)).When the bond angles in PtCuPt are constrained to 174deg in a bow conformation,similar to a crystal structure,the energy increase is only Zkcalmol~(-1).The above conclusions should extrapolate to(G(ident to)C)_n systems with other metal endgroups.