Central metal dependence of the NO+-NO- isomerism in pentacoordinate MX(CO)(NO)(PR3)(2) complexes

A metal dependence of the structure adopted by ML4(NO) complexes has been f ound in the isoelectronic OsCl(CO)(NO)(PR3)(2) and [IrCl(CO)(NO)(PR3)(2)]() complexes, in addition to the well-known effects of the nature of the co- ligands L. In the ab initio potential energy surfaces of the two complexes at the HF and MP2 levels two minima have been located, with their relative energies dependent on the nature of the central metal atom. In the case of the Os complex, a trigonal bipyramidal coordination sphere with a linear NO group is the global minimum, whereas the most stable structure for the Ir complex is square pyramidal with an apical bent NO. Finally, a trigonal bip yramidal structure with a linear NO was calculated as the optimum structure for the related complex Os(CH2)(CO)(NO)(PR3)(2).