Ground-State and Excited-State Structures of Tungsten–Benzylidyne Complexes

The molecular structure of the tungsten–benzylidyne
complex trans-W­(CPh)­(dppe)2Cl
(1; dppe = 1,2-bis­(diphenylphosphino)­ethane) in the singlet
(dxy)2 ground state and luminescent
triplet (dxy)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, (dxy)1-configured 1+, and (dxy)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)­Ln (E = O, N) compounds with analogous (dxy)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.