Transition metal alkylidene complexes. Pathways in their formation and tautomerization between bis-alkylidenes and alkyl alkylidynes

Studies from authors’ group (at the University of Tennessee) on alkylidene complexes and α-H migration in alkyl alkylidyne complexes, leading to unusual tautomerization equilibria between bis-alkylidenes and alkyl alkylidynes, are reviewed. Preparation of silyl alkylidene complexes (Me3ECH2)2Ta(double bond; length as m-dashCHEMe3)(SiR3) [R3 = (SiMe3)3, E = C, 3a, Si, 3b; R3 = ButPh2, E = C, 4a, Si, 4b] and the pathway in the formation of 3b are discussed first. Pathways in the formation of archetypical Schrock-type alkyl alkylidenes (Me3ECH2)3Tadouble bond; length as m-dashCHEMe3 (E = C, 5a; Si, 5b), including the work using Ta(CD2CMe3)5 (21-d10) to confirm that it is the precursor to (Me3CCD2)3Tadouble bond; length as m-dashCDCMe3 (5a-d7), are then considered. Tautomerization of silyl alkylidyne (Me3CCH2)2W(triple bond; length of mdashCCMe3)(SiButPh2) (6a) with bis-alkylidene (Me3CCH2)W(double bond; length as m-dashCHCMe3)2(SiButPh2) (6b) as well as (Me3SiCH2)3W(triple bond; length of mdashCSiMe3)(PR3) [R3 = Me3, 7a; Me2Ph, 8a; Me2(CH2)2PMe2 (DMPE-P), 9a] with (Me3SiCH2)2W(double bond; length as m-dashCHSiMe3)2(PR3) (R3 = Me3, 7b; Me2Ph, 8b; DMPE-P, 9b) [P refers to a dangling P atom in Me2P(CH2)2PMe2] is covered next. Finally the conversion of the tungsten phosphine tautomerization mixtures to alkyl alkylidene alkylidyne (Me3SiCH2)W(double bond; length as m-dashCHSiMe3)(triple bond; length of mdashCSiMe3)(PR3)2 [(PR3)2 = (PMe3)2, 10; (PMe2Ph)2, 11; DMPE, 12], including its pathway, is presented.