The compounds [MBr2(pyNO)(n)] (where M: Mn, Fe, Co, Ni, Cu or Zn; pyNO is pyridine N-oxide and n = 2, 3 or 6) were synthesized and characterized by melting points, elemental analysis, thermal analysis and electronic and IR spectroscopy. The enthalpies of dissolution of the adducts, metal(II) bromides and pyNo in methanol were measured and by using thermochemical cycles, the following thermochemical parameters for the adducts have been determined: the standard enthalpies for the Lewis acid/base reactions (Delta(r)H(theta)), the standard enthalpies of formation (Delta(f)H(theta)), the standard enthalpies of decomposition (Delta(D)H(theta)), the lattice standard enthalpies (Delta(M)H(theta)) and the standard enthalpies of the Lewis acid/ base reactions in the gaseous phase (Delta(f)H(theta)(g)). The mean bond dissociation enthalpies of the M(II)-oxygen bonds ((D) over bar ((M-O))) have been estimated. (C) 2003 Elsevier B.V. All rights reserved.