Premium
Electronic Structure and Chain‐Length Effects in Diplatinum Polyynediyl Complexes trans,trans ‐[(X)(R 3 P) 2 Pt(CC) n Pt(PR 3 ) 2 (X)]: A Computational Investigation
Author(s) -
Zhuravlev Fedor,
Gladysz John A.
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200400643
Subject(s) - triple bond , single bond , crystallography , platinum , chemistry , quadruple bond , bond length , antibonding molecular orbital , natural bond orbital , bond order , double bond , pi bond , stereochemistry , atomic orbital , density functional theory , computational chemistry , crystal structure , physics , electron , group (periodic table) , biochemistry , organic chemistry , quantum mechanics , polymer chemistry , catalysis
Structure and bonding in the title complexes are studied using model compounds trans,trans ‐[(C 6 H 5 )(H 3 P) 2 Pt(CC) n Pt(PH 3 ) 2 (C 6 H 5 )] ( PtC x Pt ; x =2 n =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 (Δ E =0.12 kcal mol −1 for PtC 8 Pt ). As the carbon‐chain length increases, progressively longer CC triple bonds and shorter C C single bonds are found. Whereas the triple bonds in HC x H become longer (and the single bonds shorter) as the interior of the chain is approached, the Pt C C triple bonds in PtC x Pt are longer than the neighboring triple bond. Also, the PtC bonds are shorter at longer chain lengths, but not the HC 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 CC atoms, elongating this triple bond. Charge decomposition analyses show no significant d–π* backbonding. The HOMOs of PtC x Pt can be viewed as antibonding combinations of the highest occupied π orbital of the sp‐carbon chain and filled in‐plane 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 HC x H increase with chain length; the origin of this dichotomy is analyzed. The electronic spectra of PtC 4 Pt to PtC 10 Pt are simulated. These consist of two π–π* bands that redshift with increasing chain length and are closely paralleled by real systems. A finite HOMO–LUMO gap is predicted for PtC ∞ Pt . The structures of PtC x Pt are not strictly linear (average bond angles 179.7°–178.8°), and the carbon chains give low‐frequency fundamental vibrations ( x =4, 146 cm −1 ; x =26, 4 cm −1 ). When the bond angles in PtC 12 Pt are constrained to 174° in a bow conformation, similar to a crystal structure, the energy increase is only 2 kcal mol −1 . The above conclusions should extrapolate to (CC) n systems with other metal endgroups.