STRUCTURAL AND CARBON-13 NUCLEAR RESONANCE STUDIES OF COMPOUNDS CONTAINING THE HEXACARBONYLDIIRON ENTITY. I. HEXACARBONYL(μ-(1,2,5-η-1,4,5-η)-3-OXO-1,2,4,5-TETRAPHENYL-1,4-PENTA# DIENE-1,5-DIYL)DIIRON(FE-FE)