Compounds containing polydentate ligands

The trimethylaluminum complexes of tetraaza macrocycles contain AlMe2 groups and externally coordinated AlMes molecules [416-419] tetrathia macro-cycles [420, 421] are perhaps better described as simple coordination compounds of polydentate ligands, rather than host-guest complexes. This is suggested by the short Al-N distances (ca 2.0 A) and the external coordination of the metal. 

Observations on the chelate effect and, in particular, the importance of ring size, enable predictions to be made concerning the formation of stable coordination compounds with polydentate ligands that contain a variety of individual functional groups. Work on the synthesis of organic ligands... 

The behaviour of polydentate ligands containing sulphinyl groups has received much less attention. Giesbrecht and Osorio203 have reported the coordination compounds of bivalent transition metal (Mn2+, Co2+, Ni2+, Cu2 +, Cu2+, Zn2+) perchlorates with 2,2 -sulphinyldiethanol (SDE). 

This section describes the synthesis as well as chemical and physical properties of Group 2 metal /3-diketonate compounds that also contain coordinated polydentate ligands to prevent oligomerization. 

The final part of the synthesis and properties section of this review addresses other alkaline-earth complexes containing mono-, bi-, or polydentate ligands and anionic ligands such as carboxylates, halides, thiocyanates, or nitrates, where the low degree of oligomerization may result in sufficient volatility for CVD. We have not attempted to be comprehensive in this section, but rather to present examples of the different types of compounds formed by these anionic ligands. More comprehensive coverage of this area can be found elsewhere.74170 223 224... 

Despite the multitude of well characterized boron compounds, the knowledge of azole derivatives of boron is still rather limited. For example, only three C-borylated pyrazoles are known. Although N-borylated pyrazole derivatives are considerably more abundant, until most recently these were restricted to compounds containing only four-coordinate boron. Of these, the poly(l-pyrazolyl)borate anions have been a bonanza for the coordination chemist, since the steric and electronic features of these ions render them as extremely useful (polydentate and chelating) ligands. However, recent studies of the chemistry of boron derivatives of pyrazoles have provided for some noteworthy developments and a survey of such compounds and their chemistry appears to be a timely subject. 

Overall, the formation of pentacoordinated complexes is favored by chelating and particularly by polydentate macrocyclic ligands. A search in the Cambridge Structural Database reveals that many of the structurally characterized five-coordinated Pd(II) compounds contain macrocyclic or rigid polydentate ligands, S, N, and P donor atoms being dominant. Complexes (23), (24) and (25) are typical examples. 

A large variety of polydentate ligands containing nitrogen and phosphorus can be prepared easily by the reaction of ethylenediamine or substituted ethyl-enediamines, formaldehyde, and secondary phosphines, as illustrated below. Furthermore, the scope of this reaction is very wide. It includes the compounds listed here and it is also general for amines which contain at least one N-—H bond and for phosphines which contain at least one P—H bond. 

---