Reactivity of Deprotonated Mn2(.mu.-H)(.mu.-PCyH)(CO)8: Selective Monoauration to Mn2(.mu.-AuPR3)(.mu.-PCyH)(CO)8 and Mn2(.mu.-H)(.mu.3-PCy(AuPR3))(CO)8 (R = Cy, Ph, p-C6H4F, p-C6H4OMe) and Kinetic Studies of their Conversion