Structural Dichotomy in Six-Coordinate d0 Complexes: Trigonal Prismatic (tBu3SiC⋮C)6Ta- and Octahedral (tBu3SiC⋮C)6M2- (M = Zr, Hf)

Utilization of the bulky acetylide, t Bu 3 SiC⋮C -, enabled the synthesis of several early metal polyacetylides. Addition of NaC⋮CH to t Bu 3 SiBr in dimethyl sulfoxide afforded t Bu 3 SiC⋮CH, which was deprotonated to yield t Bu 3 SiC⋮CLi. Treatment of ZrCl 4, HfCl 4, and TaCl 5 with varying amounts of t Bu 3 SiC⋮CLi gave {(THF) 2 Li( t Bu 3 SiC⋮C) 2 }Zr(C⋮CSi t Bu 3 ) 3 (THF) ( 1; THF = tetrahydrofuran), {(Et 2 O)Li( t Bu 3 SiC⋮C) 2 }Hf(C⋮CSi t Bu 3 ) 3 (OEt 2 ) ( 2 ), {Li( t Bu 3 SiC⋮C) 3 } 2 M (M = Zr, 6; Hf, 7 ), and {Li( t Bu 3 SiC⋮C) 3 }Ta(C⋮CSi t Bu 3 ) 3 ( 3 ). Metathesis of 3 with KOTf generated KTa(C⋮CSi t Bu 3 ) 6 ( 4 ) and cation sequestration of 4 with crypt 2.2.2 provided [K(crypt 2.2.2)][Ta(C⋮CSi t Bu 3 ) 6 ] ( 5 ). Single-crystal X-ray structural studies determined the structures (core symmetry) of 1 ( O h ), 2, ( O h ), 3 ( D 3 ), 5 ( D 3 ), 6 ( O h ), and 7 ( O h ). The D 3 h to D 3 twist in 3 and 5 has a steric origin, and the counterion position appears inconsequential. Origins of the structural preferences illustrated by the dichotomous twisted trigonal prismatic and octahedral cores of the d 0 hexaacetylides 5 and 6 were probed through density functional (ADF) and effective core potential (GAMESS) calculations. The structural difference results from a lessening electronic preference for the trigonal prism primarily a greater HOMO/LUMO gap upon moving from Ta to Zr, minor steric factors, and increased interligand repulsions in the dianion (VSEPR).