NONTRADITIONAL LIGANDS AND THEIR IMPACT ON COORDINATION CHEMISTRY

Werner's introduction of new geometric patterns for a number of inorganic compounds established the need for a working bonding model. The electron-pair bonding model of Lewis (and Huggins) provided a means for rationalizing most coordination compounds. On the other hand, coordination compounds of the transition metal carbonyls introduced new bonding problems. These have now been resolved through the concept of pi-acidity and back-bonding for ligands such as CO and F3P. The idea of pi-acidity for borane adducts of F3P, CO, and PF2H does not work well. A model based on a polarized sigma bond is developed here and is used to explain the following facts: (1) The shortest known P-B bond for a coordination compound containing 4-coordinate boron and phosphorus atoms is found in F3PBH3. That distance is even among the shortest for compounds containing 3-coordinate boron and phosphorus, yet this P-B bond dissociates very easily. (2) To date, all attempts to make CO or F3P adducts of BCl3 or BF3 have been unsuccessful. (3) AlCl3 will form a relatively stable F3P adduct. (4) HF2P forms a more stable adduct with BH3 than does either F3P or H3P. Compounds containing three-center coordinate bonds of the form [GRAPHICS] are described. These compounds are compared to organic compounds containing ''agostic'' hydrogens.