M(2)L(6) complexes with triple Mo-Mo and W-W bonds: Molecular topology and inverted pyramidality effect

A molecular orbital study of the model compounds [Mo-2(NH2)(6)], [Mo-2(OH)(6)], [W2H6] and [W2Cl6], indicates that the M-M bond strength should increase with increasing pyramidality (i.e., the average of the M-M-L bond angles) up to a certain angle (the turnover point), after which the trend is inverted. The topology of the metal-centered pi-type orbitals of the ML(3) fragments favors the appearance of the turnover point at a smaller angle than the minimum in energy, resulting in an inverted pyramidality effect for the experimentally attainable angles. A structural database search supports the theoretical findings: the M-M distances in triple-bonded complexes of d(3) transition metal ions in the families [Mo-2(NR(2))(6)], [W-2(NR(2))(6)], and [W-2(OR)(6)] decrease with increasing pyramidality angles, while the [Mo-2(OR)(6)] complexes present a parabolic bond distance-bond angle dependence.