Answer to Question #163560 in Organic Chemistry for Hamza Arshad

Question

Explain why Mo(PMe3)5H2 is a dihydride (contains two separate H ligands), but Mo(CO)3(PR3)2(H2) contains the dihydrogen ligand (Me = methyl, R = isopropyl).

Answer 1

Dihydrogen complex has more sigma donor character than hydride complex. For the first compound PMe₃ have no proper pi-acceptor ability. So, the compound is hydride but due to presence of CO, which has higher pi-acceptor characteristics. The second compound get stabilized.