Re-back-bonding

The Rh-Ir vector defines the z-axis, and the NN bridges define the x- and y-axes. The N and P atoms are solely electron donors, but the CO ligand involves a lot of re-back-bonding. Thus the quadrupole matrix, which has large components if there is an electric field gradient at the Ir nucleus, is rotated 45° compared with the g-matrix principal axes. 

The intensities of the infrared-active carbonyl stretching mode of the hexacarbonyl cations [Mn(CO)6] and [Re(CO)6] + have been measured (77). The absolute intensities of the hexacarbonyl cations are in the order [M(CO)6]+ < M(CO)6 < [M(CO)6]-, which corresponds to the reduction of 77-back-bonding from metal to carbonyl as the positive charge on the metal is increased. (34)... 

This implies that the n LUMO energy decreases in the same order, that is from bpy to Ar-dab and abpy. The Re —> diimine n back bonding strengthens in a similar order ... 

---