Photolytic homolysis of metal–metal bonds of some binuclear transition-metal carbonyls: an electron spin resonance investigation using spin trapping

Irradiation, using a 250-W high-pressure mercury lamp, of a solution of [Mn2(CO)10] and an excess of nitrosodurene (RNO) in CH2Cl2 at ⩽–30 °C in the cavity of an e.s.r. spectrometer produces an 18-line signal attributed to [Mn(CO)5{Ṅ(O)R}]. Similarly, aminyl oxides derived from spin. trapping of [Mn(CO)4L][L = P(OPh)3, PPh3, PMePh2, PMe2Ph, PBun3, or P(C6H11)3], [Re(CO)5], [Co(CO)3{P(OEt)3}], [Mo(η-C5H5)(CO)2L][L = CO or PPh3], or [Fe(η-C5H5)(CO)2] have been characterised by their e.s.r. solution spectra at ca. –30 °C, g= 2.005–2.01. The nitrogen hypetfine coupling [a(14N)] is in the range 1.45–1.75 mT, while a(55Mn)= 0.82–0.89, a(185,187Re)= 4.09, a(59Co)= 1.39, a(95,97Mo)= 0.40–0.50, and a(31P)= 0.92–1.67 mT for the manganese complexes and 0.45 mT for [Co(CO)3{Ṅ(O)R}{P(OEt)3}]. For photolysis of [Mn(CO)10]: (i) formation of aminyl oxide requires light in the 300–400 nm region; (ii) using PhMe as solvent, or with CH2Cl2 but RNO as a minor component, a 31-line spectrum assigned to [{R(O)N}(OC)4MnMn(CO)5]+· is obtained. A mechanism is proposed whereby initial formation of [Mn(CO)5] is followed by competitive processes leading either to [Mn(CO)5{Ṅ(O)R}] or [Mn(CO)5]– and [Mn2(CO)10]+·. which finally may disproportionate to MnII–Mn–I or react with RNO to yield the binuclear metal aminyl oxide.