[MF₄(dmso)₂] (M = Zr or Hf) and [MF₄(dmf)₂], prepared by dissolving MF₄·nH₂O in the appropriate solvent, have been used as synthons for a range of complexes of these otherwise intractable tetrafluorides. These reagents react with OPR₃ (R = Me or Ph) or OAsPh₃ (L) in anhydrous CH₂Cl₂ to form six-coordinate [MF₄L₂] which exist as a mixture of cis (predominant form) and trans isomers in CH₂Cl₂ solution but which crystallise as trans (OPPh₃, OAsPh₃) or cis (OPMe₃) forms. Cis-[ZrF₄(OAsPh₃)₂] crystals were obtained from MeCN. Cis-[MF₄(pyNO)₂] and eight-coordinate (distorted dodecahedral) [MF₄(L–L)₂] (L–L = 2,2′-bipy, or 1,10-phen), and [MF₄(Me₄-cyclam)] were also obtained. Attempts to prepare complexes with the N-heterocyclic carbene, 1,3-(2,6-di-isopropylphenyl)imidazol-2-ylidene (IDiPP) or alkyl diphosphines were unsuccessful. Crystal structures are reported for trans-[ZrF₄(OPPh₃)₂], cis- and trans-[ZrF₄(OAsPh₃)₂], cis-[HfF₄(OPMe₃)₂], [ZrF₄(2,2′-bipy)₂], cis-[HfF₄(dmf)₂], and geometric isomers (both pentagonal bipyramidal) of [(dmso)₂F₃M(μ-F)₂MF₃(dmso)₂]. The failed attempts to make IDiPP adducts led to crystals of [IDiPPH]₃[M₃F₁₅] containing discrete anions based upon a triangle of M atoms with single F bridges. The results are compared with previous work on TiF₄ adducts and with complexes of MCl₄, and demonstrate that the MF₄ are very hard Lewis acids, with a marked preference for O- over N-donors.