A triad of electron-rich .mu.2-alkylidene complexes M2(CO)6[.mu.-CHC(Ph)NEt2](.mu.-PPh2) (M = Fe, Ru, Os) via the addition of diethylamine to the acetylides M2(CO)6(.mu.2-.eta.2-C.tplbond.CPh)(.mu.-PPh2): spectroscopic properties and structural chemistry