Synergic pairs, transition metal

P-donor ligands usually form phosphorus-metal bonds with a main group or transition metal that are essentially covalent and in which the phosphorus is the lone pair donor. Despite the vast number of complexes prepared, often with elaborate architecture, the nature of the phosphorus-metal bond remains elusive and controversial, in particular, for transition metals. The simple statement of the bond as a neutral donor-acceptor complex has been elaborated by identifying the many contributing factors. We will discuss the steric factor and the electronic component of the P-M bond separately. The separation of the electronic component into a and n factors is a formal exercise, the reality being Synergic Bonding, wherein these electronic factors are mutually dependent. 

Crystal structure plays a secondary role in catalysis by the Transition Metal Sulfides. As the periodic trends for HDS of the binary sulfides shows the dominant effect is which transition metal is present in the reaction, this transition metal takes on the structure and stoichiometry of the phase which is most stable in the sulfur containing catalytic environment. The unsupported promoted catalyst systems can be grouped into "synergic" pairs of sulfides. Because these pairs are related to the basic periodic trends of the binary Transition Metal Sulfides through average heats of formation. 

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