Laser-ablated Zr, Hf, and Th atoms react with NO to form the bent NMO insertion products. These favorable reactions proceed on annealing to 21-24 K. The NMO molecules are characterized by isotopic substitution and M-O stretching modes at 844.2, 855.2, and 760.3 cm(-1) and M-N stretching modes at 673.3, 685.3, and 697.3 cm(-1), respectively. The increase in bond stretching frequencies from Zr to Hf is due to relativistic contraction not found from Hf to Th. The BP86 functional and averaged relativistic effective potentials for Zr and Hf predict bent (2)A' states and observed frequencies within 1-3% and the isotopic shifts within 2 cm(-1).