Gravar-mail: [1,2-Bis(diphenyl­phosphan­yl)ethane-2κ(2) P,P′]tetra­carbonyl-1κ(3) C,2κC-(μ-2-cyclo­pentyl-2-aza­propane-1,3-dithiol­ato-1:2κ(4) S,S′:S,S′)diiron(II)(Fe—Fe)