Coordination numbers, nitrogen

Phosphorus compounds exhibit an enormous variety of chemical and physical properties as a result of the wide range ia the oxidation states and coordination numbers for the phosphoms atom. The most commonly encountered phosphoms compounds are the oxide, haUde, sulfide, hydride, nitrogen, metal, and organic derivatives, all of which are of iadustrial importance. The hahde, hydride, and metal derivatives, and to a lesser extent the oxides and sulfides, are reactive iatermediates for forming phosphoms bonds with other elements. Phosphoms-containing compounds represented about 6—7% of the compound hstiugs ia Chemical Abstracts as of 1993 (1). 

The i5p-titanium(IV) atom is hard, ie, not very polarizable, and can be expected to form its most stable complexes with hard ligands, eg, fluoride, chloride, oxygen, and nitrogen. Soft or relatively polarizable ligands containing second- and third-row elements or multiple bonds should give less stable complexes. The stabihty depends on the coordination number of titanium, on whether the ligand is mono- or polydentate, and on the mechanism of the reaction used to measure stabihty. 

By far the most important metal containing dyes are derived from OjO-dUiydroxyazo stmctures in which one of the two azo nitrogen atoms and the two hydroxyl oxygen atoms are involved in bonding with the metal ion. Thus these dyes serve as terdentate ligands. In the case of metal ions with a coordination number of four, eg, Cu(H), the fourth position is usuaUy occupied by a solvent molecule (47). 

Acetylpyridine thiosemicarbazone forms [Co(8)2Cl2], which is isolated from hot ethanol [178], Based on infrared spectra the pyridyl nitrogen is coordinated and bonding is NS with two chlorines bringing the coordination number to six. The complex is a non-electrolyte in DMF, has a magnetic moment of 4.13 B.M., and the electronic spectrum has bands at about 8160 and 17 860 cm consistent with octahedral stereochemistry. 

Mononuclear silver(I) complexes with nitrogen ligands can have several coordination numbers, the simplest is dicoordinated and compounds of the type [AgL2]+ occur for a variety of ligands... 

Although there are a relatively large number of N(+V) molecules, the absence of any with a coordination number greater than four must be attributed to the small size of the nitrogen... 

---