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Abstract
Thermal substitution reactions of M(CO)5Br (M=Mn, Re) with cis (L c ) and trans (L t ) diazaphosphole ligands (L=3,4-dihydro-2,3,4,5-tetraphenyl-2H-1,2,3-diazaphosphole) have been studied. It has been found that L c behaves as generally found for bulky phosphines affording to cis-M(CO)4(Lc)Br, mer-M(CO)3(L c )2Br and fac-Re(CO)3(L c )2Br which has been thermally isomerized to mer-Re(CO)3(L c )2Br. In contrast the corresponding reaction with the bulkier trans diazaphosphole isomer L t have only afforded cis-M(CO)4(L t )Br and mer-M(CO)3(L t )2Br. All the different stereochemical results are rationalized on the basis of the size of the central metals and of the steric demand of the ligands. The nature of the complexes has been established essentially by IR spectra which, in the case of cis-Re(CO)4(L c )Br presents in cyclohexane solution a doubling of all the characteristic v(CO) bands expected for the C 2v local symmetry. This unexpected result has been tentatively discussed also on the light of an X-ray study which has confirmed the cis nature of this complex.