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- (42-), which was isolated as the bright yellow dipotassium salt [K(diglyme)]2-4. Single crystal X-ray diffraction analysis revealed a centrosymmetric structure of 42-. The Mo-Si bond length of 42- (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 42- shed light into the electronic structures of these compounds.