Proximity-assisted formation of bridged iron and ruthenium carbonyl complexes of .mu.-polymethylene bis(1,3-pentadiene). Preparation and molecular structure of Ru(CO)[(.mu.-(CH2)3)(.eta.4-C5H7)2] and (.mu.-(CH2)3)[Ru(CO)3(.eta.4-C5H5)][Ru(CO)(.eta.4-C5H7)] (Ru-Ru)