Reactions of in situ generated [Li(Giso)] (Giso = [(DipN) 2 CN(C 6 H 11 ) 2 ] -; Dip = C 6 H 3 Pr 2 i -2,6) with either ZrCl 4 or HfCl 4 led to the complexes [(Giso)MCl 3 ] (M = Zr 1 or Hf 2). Reduction of [(Giso)ZrCl 3 ] with an excess of magnesium metal in THF afforded a small isolated yield of the THF insertion product, [{(Giso)ZrCl(l-OC 4 H 8 )} 2 ]. The crystal structures of all complexes have been determined. The cell parameters of the compounds are 1 (toluene) 0.5 : P-1, a = 10.961(2), b = 11.046(2), c = 19.668 (4), a = 75.31(3), b = 76.69(3), c = 61.93(3); 2 (toluene) 0.5 : P-1, a = 10.949(2), b = 11.037(2), c = 19.621(4), a = 75.29(3), b = 76.80(3), c = 61.90(3); 3 (hexane) 2 : P2 1 /c, a = 14.424(3), b = 16.950(3), c = 20.330(4), b = 109.09(3). The crystal structures of 1 and 2 are the first for fivecoordinate, monomeric guanidinato group 4 metal trihalide complexes.