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

Abstract

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 Mn^II–Mn^–I or react with RNO to yield the binuclear metal aminyl oxide.