Photochemical formation and stereochemical characterization of two electronically equivalent but structurally different series of triangular dicobalt-metal clusters, MCo2(.eta.5-C5Me5)2(.mu.-CO)3(.mu.3-CO) [M = Cr(.eta.6-C6H5Me), Mn(.eta.5-C5H4Me), Fe(.eta.4-C4H4)] and MCo2(.eta.5-C5Me5)2(.mu.-CO)2(.mu.3-CO) [M = Fe(CO)3, Co(.eta.5(C5H4Me)]: a rational synthesis of mixed-metal clusters by photogenerated metal-fragment addition across a metal-metal multiple-bonded dimer