Synthesis of a tetrametallic acetylene. X-ray structures of [{(CO)(3)Mo(mu-S2CPCy3)Mn(CO)(3)}(2)-(mu(4)-eta(1):eta(1):eta(1 '):eta(1 ')-C-2)] and [[(CO)(3)Mo(mu-CCPh)(mu-S2CPCy3)Mn(CO)(3)]