STRUCTURAL DICHOTOMY IN 6-COORDINATE D(0) COMPLEXES - TRIGONAL PRISMATIC ((BU3SIC)-BU-T-C)(6)TA- AND OCTAHEDRAL ((T)BU(3)SIC-C)(6)M2- (M = ZR, HF)

Utilization of the bulky acetylide, (Bu3SiC)-Bu-t=C-, enabled the synt hesis of several early metal polyacetylides. Addition of NaC=CH to (Bu 3SiBr)-Bu-t in dimethyl sulfoxide afforded (Bu3SiC)-Bu-t=CH, which was deprotonated to yield (Bu3SiC)-Bu-t=CLi. Treatment of ZrCl4, HfCl4, a nd TaCl5 with varying amounts of Bu-t(3)-SiC=CLi gave Li((Bu3SiC)-Bu-t =C)(2)}Zr(C=(CSiBu3)-Bu-t)(3)(THF) (1; THF = tetrahydrofuran), )Li(Bu- t(3)-SiC=C)(2)}Hf(C=(CSiBu3)-Bu-t)(3)(OEt2) (2), {Li((Bu3SiC)-Bu-t=C)( 3)}(2)M (M = Zr, 6; Hf,7), and {Li((Bu3SiC)-Bu-t=C)(3)}Ta(C=(CSiBu3)-B u-t)(3) (3). Metathesis of 3 with KOTf generated KTa(C=(CSiBu3)-Bu-t)( 6) (4) and cation sequestration of 4 with crypt 2.2.2 provided [K(cryp t 2.2.2)][Ta(C=(CSiBu3)-Bu-t)(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-3h to D-3 twist in 3 and 5 has a steric origin, and the counterion position appears inconse quential. Origins of the structural preferences illustrated by the dic hotomous twisted trigonal prismatic and octahedral cores of the d(0) h exaacetylides 5 and 6 were probed through density functional (ADF) and effective core potential (GAMESS) calculations. The structural differ ence results from a lessening electronic preference for the trigonal p rism-primarily a greater HOMO/LUMO gap-upon moving from Ta to Zr, mino r steric factors, and increased interligand repulsions in the dianion (VSEPR).