Synthesis and Characterization of Novel M−Hg−M (M = Cr, Mo, W) Heterotrimetallic Complexes Bridged by a Functionally Substituted Bis(cyclopentadienyl) Ligand. Crystal and Molecular Structure of [Mo(CO) 3 (η 5 -C 5 H 4 CH 2 CH 2 OCH 2 CH 2 C 5 H 4 -η 5 )(CO) 3 Mo]Hg