Stereochemistry of Transition-Metal Dinitrosyl Complexes. A Molecular Orbital Rationale for the Attracto and Repulso Conformations.

Transition-metal dinitrosyl complexes constitute a fairly large class of compounds, exemplified by some 500 structures in the Cambridge Structural Database. While many of the complexes exhibit a claw-like cis-attracto conformation, a handful of them exhibit a peculiar repulso conformation, in which the two NO groups are splayed outward and away from each other. Surprisingly, no computational study to date has attempted to explain the existence of these two limiting conformations of cis-dinitrosyl complexes. Careful examination of the large body of structural data and density-functional-theory-based molecular orbital analyses has identified both specific Enemark-Feltham electron counts and metal-ligand orbital interactions as crucial to each of the two conformations. Thus, the common attracto conformation, which is favored by as many as four metal(d)-NO(π*) orbital interactions, is observed most characteristically in four- and five-coordinate cis-{M(NO)} complexes. The rarer repulso conformation, characterized by an unusually wide NMN' angle, appears to be typical of pseudotetrahedral {M(NO)} complexes involving 4d and 5d transition metals. These complexes exhibit an a-symmetry (under a C molecular point group) metal(d)-NO(π*) orbital interaction that uniquely favors a repulso geometry. This orbital interaction, however, appears to be weaker for 3d orbitals, which are significantly smaller than 4d and 5d orbitals. Pseudotetrahedral {M(NO)} complexes involving a 3d transition metal accordingly thus tend to exhibit an attracto conformation for hard, nitrogen-based supporting ligands but repulso-like/borderline conformations for soft phosphine- and thioether-type supporting ligands.