The nickel(II) pincer complexes ( 1 : R = Ph, X = H; 2 : R = Ph, X = Me) undergo differe...

The nickel(II) pincer complexes ( 1 : R = Ph, X = H; 2 : R = Ph, X = Me) undergo different reactions with CO on the basis of the R and X groups, but the phosphine arms remain attached to the Ni center (See L.-C. Liang, Y.-T. Hung, Y.-L. Huang, P.-Y. Lee, W.-C. Chen, Organometallics , 2012 , 31 , 700 for structural details of these complexes.)

a. When R = Ph and X = H, the addition of CO results in the disappearance of a ¹ H NMR resonance at –18 ppm, with the corresponding new signal (that also integrates to one H) at 8.61 ppm. Two v(CO) stretching bands appear in the IR spectrum at 1993 and 1924 cm^-1 . Propose a structure for this product and a mechanism for its formation.

b. When R = Ph and X = Me, the addition of CO results in the appearance of an intermediate with a single v(CO) infrared band at 1621 cm^-1 . This intermediate subsequently provides another nickel complex with three v(CO) infrared bands (2002, 1943, 695 cm^-1 ). Propose the structures of the intermediate and final product, and the reaction mechanisms involved in these reactions.

c. Why do the reactions proceed differently when X = H or X = Me?