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 (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 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.