Calculations reported on the fragments Mn(CO)5, Mn(CO)3, Cu(PH3), and Au(PH3) have shown...

Calculations reported on the fragments Mn(CO)₅, Mn(CO)₃, Cu(PH₃), and Au(PH₃) have shown that the energies of their singly occupied hybrid orbitals are in the order Au(PH₃) > Cu(PH₃) > Mn(CO)₃ > Mn(CO)₅.

a. In the compound (OC)₅MniAu(PH₃), would you expect the electrons in the Mn–Au bond to be polarized toward Mn or Au? Why? (Hint: sketch an energy-level diagram for the molecular orbital formed between Mn and Au.)

b. The Cu(PPh₃) fragment bonds to C₅H₅ in a manner similar to the isolobal Mn(CO)₃ fragment. However, the geometry of the corresponding Au(PPh₃) complex is significantly different:

Suggest an explanation. (See D. G. Evans, D. M. P. Mingos, J. Organomet. Chem. , 1982 , 232 , 171.)