Electronic communication between tungsten alkylidyne and metal isocyanide complex fragments across phenyleneethynylene bridges

Heteronuclear metal complexes of the type [X(CO)₂(LL)WC(-C₆H₄-CC)_p-C₆H₄-NC-]_nML_m (X = Cl, Br; LL = tmeda, dppe; p = 0-3; n = 1 for Cr(CO)₅, n = 2 for ReX(CO)₃, PdCl₂, PdI₂, and PtI₂) have been prepared. The molecular structure of one example, [Cl(CO)₂(tmeda)WC-C₆H₄-NC-]₂PdI₂, was determined by X-ray crystallography. The extent of electronic communication between the tungsten alkylidyne and the metal isocyanide centers was probed by various spectroscopic techniques. In the parent systems (p = 0), the electronic changes due to modification of the isocyanide metal complex fragments could be distinguished clearly by the ¹³C NMR chemical shift of the alkylidyne carbon atom and the d → π^∗ and π → π^∗ electronic transitions of the metal alkylidyne system. However, only residual effects could be discerned for the longer systems using these spectroscopic probes. Probes based on the emission property of the tungsten alkylidyne fragment proved to be the most useful in distinguishing long-range effects in those cases where the isocyanide metal complex fragment is capable of quenching the emission. This is the case for ML_m = PdCl₂ and PdI₂. Partial quenching effects were still observed at the longest investigated distance between the metal centers of about 3.1 nm. The available evidence suggests that the quenching mechanism is electron transfer.