1,2-diamidolenes, -diphosphidolenes, and -dithiolenes as riders on sawhorses (L6M2 units). A theoretical interpretation of the stereochemistries, residual bonding capabilities, and contrasts to the behavior of 1,2-dioxolenes

This paper analyzes from the theoretical viewpoint (ehmo calculations with the graphic interface of CACAO) the bonding capabilities of conjugated pi-ligands containing the unit XCCX (X = S, NR, PR) when they ride on binuclear fragments of the type L6M2 (sawhorse). The stereochemistries are monitored for a number of structures retrieved from the Cambridge Database. In the starting prototype, e.g. the complex [(CO)(2)(PPh3)Ru{mu-1,2-(NH)(2)C6H4}Ru(PPh3)(CO)(2)] with 34 valence electrons, the rider donates 4 electron pairs to two d(7) metals and a single M-M bond is featured. In spite of the predicted electron similarities between diamidolenes and dioxolenes, no case has been reported of a catecholate ligand adapting to the same riding mode and therefore being able to donate a total of 4 lone pairs to the metals, The different behavior is largely ascribed to poor interactions of b(1) symmetry when the donor atoms of the rider are of higher electronegativity. Stronger donors, such as phosphorus and sulfur, still support the riding mode but elongate understandably the M-M bond. In presence of d(6) metal atoms, the riding ligand bends over one of the metals and also donates the electrons of the C=C pi-bond itself. Alternatively, an extra ligand (terminal or bridging) can be added to the primary framework M2L6. In all of the latter cases, the various factors affecting the nature of the M-M bond are theoretically interpreted.