Many new compounds of chromium(II), vanadium(II), and vanadium(III) have been prepared under anaerobic conditions and investigated by spectroscopic, magnetic susceptibility, and X-ray powder and single crystal techniques. New tetrahalogenochromates(II) containing mixed halides of the type [o-CH3C6H4NH3]2[CrClxBr4-x(B) ] (where x = 4, 3, 2, B = CH3CO2H; x = 4, 1, 0, B = C2H5CO2H; x = 4, B = C4H8O) [NH4 ]2 [ CrClxBr4-x] nCH3CO2H (where x = 3, n = 1; x = 0, n = 0. 5), [(CH3)2NH2]2[CrCl2Br2]CH3CO2H, [C6H5(CH2)2NH2(CH3)]2[CrCl4], [C6H5NH3]2[CrClxBr4-x] (x=2, 0. 5), [C6H5CH2NH3]2[CrxBr4-x] (x= 3. 3, 2. 2, 1, 0) and the trihalogenochromate(II) [C6H5NH3] [CrCl3] have been characterised. The o-methylanilinium salts are all magnetically-normal, high-spin chromium(ll) compounds. The ammonium, dimethylammonium, N-methylphenethylammonium and anilinium salts are antiferromagnetic, whereas all benzylammonium salts exhibit typical ferromagnetic behaviour. Reflectance spectra are as expected for tetragonally distorted octahedral chromium(II) compounds. Thus, from the magnetic and spectral results halide-bridged polymeric structures have been assigned except to the o-methylanilinium salts which probably contain square-pyramidal anions. Single crystal X-ray investigations show that [C6H5NH3][CrCl3] consists of linear chains of face-sharing distorted [CrCl6] octahedra, and in [C6H3CH2NH3]2[CrBr4] each square planar [CrBr4]2- unit is bridged by bromide ions to four like units to form layers. A series of halide adducts of tetrakis(acetato)dichromium(II) of the type [A]4[Cr2(CH3CO2)4X2]X2 (where A = NH4, C6H5NH3, C6H5CH2NH3, X = Cl or Br; A = 0. 5C4H12N2, C6H5(CH2)2NH2(CH3), X = Cl; A =C5H5NH, X = Br) and [A] 2[Cr2(CH3CO2)4Cl2] (A = C(NH2)3, C5H5NH) has been prepared. All are almost diamagnetic and retain the acetato-bridged dinuclear structure. The di-tertiary phosphine complexes [CrX2(dmpe)2] X = Br or I; dmpe = bis(1,2-dimethylphosphino)ethane, [CrX2(depe)2]X = Cl, Br or I; depe = bis(1,2-diethylphosphino)ethane, are low spin except trans-[Crl2 (depe) 2], which is unusual in that it is high spin at room temperature, but undergoes a sharp transition to the low spin form at ca. 170K. It is the first example of spin-crossover in a chromium(ll) complex. The known complexes of acetonitrile [CrX2(MeCN)2] (X = Cl or Br) have halide-bridged linear polymeric structures instead of the distorted tetrahedral structure reported earlier. Both are weakly antiferromagnetic. Chromium(ll) complexes with triphenylphosphine oxide of the type [CrX2(Ph3PO)2] (X = Cl, Br or I), [CrX2(Ph3PO)2(THF)2] (X = Br or I) have been re-prepared. The complex [CrBr2(Ph3PO)2] is trans planar and not tetrahedral as proposed earlier and it is isomorphous with [CrCl2(Ph3PO)2], but not with [Crl2(Ph3PO)2]. Some crystals obtained during the preparation of the iodo-complex were found by X-ray investigation to consist of trigonal bipyramidal [Crl2(Ph3PO)3]. Novel series of complex halides of vanadium(ll) [A]2[VCI4(CH3CO2H)2] and vanadium(III), [A]4[VCl6]Cl and [A]2[VCI5(B)] (where A is an organic cation and B is CH3CO2H or C2H5CO2H) have been found to be magnetically normal, octahedral complexes. The structure has been confirmed by an X-ray investigation of [MeNH3]4[VCl6]Cl. Some new vanadium(II) complexes with aromatic diamines of the type V(L)4X2, X = Cl or Br; L= o-phenylenediamine or 4-methyl-o-phenylenediamine have been characterised.