Linearly Two-Coordinated Silicon: Transition Metal Complexes with the Functional Groups MSiM and MSiM

A detailed experimental and theoretical analysis is presented of unprecedented molybdenum complexes featuring a linearly coordinated, multiply bonded silicon atom. Reaction of SiBr2(SIdipp) (SIdipp = C­[N­(C6H3-2,6-iPr2)­CH2]2) with Na­[Tp′Mo­(CO)2(PMe3)] (Na-1) in the ratio 1:2 afforded the reddish-brown metallasilylidyne complex [Tp′(CO)2MoSiMo­(CO)2(PMe3)­Tp′] (Tp′ = κ3-N,N′,N″-hydridotris­(3,5-dimethylpyrazolyl)­borate) (2), in which an almost linearly coordinated silicon atom (∠(Mo1–Si–Mo2) = 162.93(7)°) is bridging the 15VE metal fragment Tp′Mo­(CO)2 with the 17VE metal fragment Tp′Mo­(CO)2(PMe3) via a short Mo1–Si bond (2.287(2) Å) and a considerably longer Mo2–Si bond (2.438(2) Å), respectively. The reddish-orange silylidyne complex [Tp′(CO)2MoSiTbb] (3) was also prepared from Na-1 and the 1,2-dibromodisilene (E)-Tbb­(Br)­SiSi­(Br)­Tbb (Tbb = C6H2-2,6-[CH­(SiMe3)2]2-4-tBu) and contains as 2 a short Mo–Si bond (2.2614(9) Å) to an almost linearly coordinated Si atom (∠(Mo–Si–CTbb) = 160.8(1)°). Cyclic voltammetric studies of 2 in diglyme revealed an irreversible reduction of 2 at −1.907 V vs the [Fe­(η5-C5Me5)2]+/0 redox couple. Two-electron reduction of 2 with potassium graphite yielded selectively the 1,3-dimetalla-2-silaallene dianion [Tp′(CO)2MoSiMo­(CO)2Tp′]2– (4 2–), which was isolated as the bright yellow dipotassium salt [K­(diglyme)]2-4. Single crystal X-ray diffraction analysis revealed a centrosymmetric structure of 4 2–. The Mo–Si bond length of 4 2– (2.3494(2) Å) compares well with those of Mo–Si double bonds and lies in-between the Mo1–Si triple bond and Mo2–Si single bond length of 2. Compounds 2, 3 and [K­(diglyme)2]-4 were characterized by elemental analyses, IR and multinuclear NMR spectroscopy. Comparative ELF (electron localization function), NBO (natural bond orbital) and NRT (natural resonance theory) analyses of 2, 3 and 4 2– shed light into the electronic structures of these compounds.