title: Ground-state and excited-state structures of tungsten-benzylidyne complexes.
abstract: The molecular structure of the tungsten-benzylidyne complex trans-W(≡CPh)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) in the singlet (d(xy))(2) ground state and luminescent triplet (d(xy))(1)(π*(WCPh))(1) excited state (1*) has been studied using X-ray transient absorption spectroscopy, X-ray crystallography, and density functional theory (DFT) calculations. Molecular-orbital considerations suggest that the W-C and W-P bond lengths should increase in the excited state because of the reduction of the formal W-C bond order and decrease in W→P π-backbonding, respectively, between 1 and 1*. This latter conclusion is supported by comparisons among the W-P bond lengths obtained from the X-ray crystal structures of 1, (d(xy))(1)-configured 1(+), and (d(xy))(2) [W(CPh)(dppe)(2)(NCMe)](+) (2(+)). X-ray transient absorption spectroscopic measurements of the excited-state structure of 1* reveal that the W-C bond length is the same (within experimental error) as that determined by X-ray crystallography for the ground state 1, while the average W-P/W-Cl distance increases by 0.04 Å in the excited state. The small excited-state elongation of the W-C bond relative to the M-E distortions found for M(≡E)L(n) (E = O, N) compounds with analogous (d(xy))(1)(π*(ME))(1) excited states is due to the π conjugation within the WCPh unit, which lessens the local W-C π-antibonding character of the π*(WCPh) lowest unoccupied molecular orbital (LUMO). These conclusions are supported by DFT calculations on 1 and 1*. The similar core bond distances of 1, 1(+), and 1* indicates that the inner-sphere reorganization energy associated with ground- and excited-state electron-transfer reactions is small.
authors.affiliation: Department of Chemistry, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, USA.
authors.forename: Benjamin M
authors.initials: BM
authors.lastname: Lovaasen
authors.forename: Jenny V
authors.initials: JV
authors.lastname: Lockard
authors.forename: Brian W
authors.initials: BW
authors.lastname: Cohen
authors.forename: Shujiang
authors.initials: S
authors.lastname: Yang
authors.forename: Xiaoyi
authors.initials: X
authors.lastname: Zhang
authors.forename: Cheslan K
authors.initials: CK
authors.lastname: Simpson
authors.forename: Lin X
authors.initials: LX
authors.lastname: Chen
authors.forename: Michael D
authors.initials: MD
authors.lastname: Hopkins