Halogen oxidation of complexes of the type cis-[M(CO)4L2], (M = Mo or W; L = PEt3, PMe2Ph, or AsMe2Ph) gives seven-co-ordinate complexes, [MX2(CO)3L2](X = Cl, Br, or I). The yellow complex [MoBr2(CO)3(PEt3)2] loses carbon monoxide reversibly to give a blue complex [MoBr2(CO)2(PEt3)2]. Chlorine oxidation of cis-[Mo(CO)4(PMe2Ph)2] gives [Mo2Cl2(CO)4(PMe2Ph)4] and the complexes [MoX2(CO)3(PMe2Ph)2](X = Br or I) on heating readily give [Mo2X2(CO)4(PMe2Ph)4]. Halogen oxidation of complexes of the type fac-[M(CO)3L3] generally gives unidentified products. Treatment of [WBr2(CO)4] with PMe2Ph (two mole per tungsten atom) gives [WBr2(CO)3(PMe2Ph)2]. Halogenocarbonyls of type [MX2(CO)4](M = Mo or W; X = Cl or Br) with three moles of PMe2Ph give complexes of type [MX2(CO)2(PMe2Ph)3]. Treatment of complexes of type cis-[M(CO)4L2](M = Mo or W; L = PMe2Ph or PEt2Ph) with an excess of bromine or chlorine (X2) gives complexes of type trans-[MX4L2]. The paramagnetic complexes [WX4(PMe2Ph)2](X = Cl or Br), take up one mole of PMe2Ph reversibly to give paramagnetic seven-co-ordinate complexes of type [WX4(PMe2Ph)3]. The magnetic moments of the compounds trans-WCl4(PMe2Ph)2] and [WCl4(PMe2Ph)3],½C6H6 over the temperature range 90–300°K are given and discussed. The 1H n.m.r. spectra of these tungsten(IV) complexes show narrow lines and the resonances have abnormal chemical shifts owing to contact and/or pseudocontact interactions of the unpaired electrons. N.m.r. and i.r. data [ν(CO) and ν(M–Cl)] are given and discussed.