Terminal phosphido complexes Cp(2)M(H)PPh(2) (1, M = Mo,W) react with group 6 metal carbonyls M’(CO)(5)-(THF) (M’ = Cr, Mo, W) leading to the monophosphido-bridged compounds Cp(2)M(H)(mu-PPh(2))M’(CO)(5) (2) and Cp(2)M(mu-PPh(2),H)M’(CO)(4) (3) in high yields. Complexes 3 are also easily obtained in reactions of 1 with M’(CO)(4)L(2) (L(2) = nbd or pip(2)). New complexes 2 and 3 were studied by IR, H-1 and P-31 NMR spectroscopies, and EHMO calculations were performed on their models, as well as on that of metallophosphines 1. A very good agreement of results obtained by different methods is observed, in particular in that which concerns the fluctuation of electron density between different molecular fragments. These results show that M’(CO)(4) fragments in complexes 3 are able to accept more electron density than the M’(CO)(5) units in complexes 2. The W-W distance of 3.271(1) Angstrom found by X-ray analysis of Cp(2)W(mu-PPh(2),H)W(CO)(4) is considered as indicative of a metal (18e(-))-metal (18e(-)) interaction. The electronic nature of the "transition metal gauche effect" is discussed on the basis of EHMO calculations.