Dimolybdenum complexes (X) (Y) [(η5-C5H3)Mo(CO)3]2 [Y=Me2Si, X=Me2C, (CH2)5C, CH2; Y=Ph2Si, X=Me2C] and desilylation products (X)[(η5-C5H4)Mo(CO3)2 were obtained from the reactions of carbon and sliicon doubly bridged biscyclopentadiene ligands (X) (Y) (C5H4)2 with Mo(CO)6 in refluxing xylene. When these ligands reacted with W(CO)6, under similar conditions, in addition to ditungsten complexes (X) (Y) [(η5-C5H3)W(CO)3]2 [X=Me2C; Y=Me2Si, Ph2Si] and desilylation products (X) [(η5-C5H4)W(CO)3]2, a class of structurally novel comolexes (X) (η5-C5H3) [(Y)W(CO)3][W(CO)3][for X=Me2C, (CH2)5C; Y=Me2-Si] with a W-Si bond were also isolated. The reactions of carbon and germanium dobuly bridged biscyclopentadiene ligands (X)(Me2Ge)(C5H4)2[X=Me2C, (CH2)5C] with Mo(CO)6 or W(CO)6 gave similar novel complexes (X)(η5-C5H3)(η5, η1-C5H3)[(Me2Ge)M(CO)3][M(CO)3][X=Me2C, (CH2)5C; M=Mo, W] with a M-Ge bond. The reactions also produced degermylation products. However, the corresponding dimolybdenum or ditungstem complexes were not isolated in these cases. The molecular structures of 2, 6, 8, 10, 11, 13, 14, 15, 19, 21, 22, 25t, and 26t have benn determinde by X-ray diffraction. The M-M distances in complexes 2 [3.4328(12)A], 8 [3.453(2)A], and 11 [3.403(2) A] are conspicuously elongated and are in fact the longest ever reported among the biscyclopentadienyl dimolybdenum or ditungsten complexes. The factors affecting the structures of dimolybdenum or ditungsten complexes are discussed.