Synthesis and reactivity studies of the stable bimetallic formyl complexes [(CO)4W(.mu.-PPh2)2Os(CHO)(CO)2(PR3)]-. Crystal and molecular structure of a methylation product [cyclic]-(CO)4W(.mu.-PPh2)[.eta.2(P,C)-.mu.-PPh2CHOMe]Os(CO)2(PMePh2) which derives by coupling of phosphido and carbene ligands