Coordination compounds effective atomic number rule

Although Mo (II) and W(II) exhibit the most extensive seven-coordinate chemistry yet known (consistent with the application of the effective atomic number rule to these systems), a survey of the substituted metal carbonyl complexes of molybdenum and tungsten reveals no structural data for compounds of the type M(00)3(6-6)2 where 6-6 is a bidentate monoanionic ligand. No tungsten tricarbonyls of this type have been reported to date, but McDonald and co-workers have synthesized and studied the closely related W(CO)2(PPh3) (6-6)2 compounds where 6-6 is a chelating dithiocarbamate (7), xanthate, or dithiophos-phate (8). For molybdenum, the compounds Mo(CO)3(dtc)2(dtc = R2NCS2") (9) are examples of the M(CO)3(6-6)2 type as is Mo(CO)3-[S2P(i-Pr)2]2 (10), which has been well characterized in solution. 

For the transition metal complexes possessing the coordination number w, the number of bonding and nonbonding orbitals equals nine and, in the case of the main group compounds, the number of these orbitals is four. This leads to the effective atomic number rule (Sidgwick s rule) and the octet rule, respectively. In and EL ... 

Consideration of the effective atomic number rule and the determination of coordination number by the electron-donating properties of ligands has led to the arranging of compounds in isostructural and/or isoelectronic... 

---