Chelate ring modes

The coordination modes found in Ni11 complexes with 2-(diphenylphosphino)nicotinic acid and its methyl ester and 2-(diphenylphosphino)benzoic acid and its methyl ester are surprising.696 The complex with the benzoic acid derivative shows the expected P,0 coordination with a six-membered chelate ring (252), while the nicotinic acid analogue forms a four-membered chelate ring with P,N binding (251). 

Carboxylates exhibit diverse coordination modes (Fig. 9) from monodentate, where the ligand is part of a chelate ring (Fig. 9, top left), through symmetrical or asymmetrical bidentate, to bridging via syn and/or anti linkages. 

After the ESIPT the molecule exhibits a pronounced ringing in this mode. The 295 cm-1 mode is a symmetric in-plane stretching vibration (see Fig. 3). The corresponding contraction of the molecule reduces the donor acceptor distances in both chelate rings simultaneously and initiates the electronic configuration change of the concerted ESIPT. The concerted double proton transfer leads therefore to a ringing of the molecule in this second mode. 

The chelating bonding mode is less common for triazenido and amidino ligands than the bridging mode. A factor may be the steric bulk of the ligand, but another factor is clearly the strain involved in the formation of a four-membered ring, which is clear from the acute NYN angle of about 102-105° (Table 3). 

Several examples are known of N—O coordination of iV-hydroxypiperidines resulting in three-membered chelate rings on deprotonation (14). The crystal structures and Mo NMR studies of some complexes of MoVI containing this grouping have appeared.46"4 A similar coordination mode was observed in the crystal structure of a Pdn complex of the 2,2,4,4-tetramethylpiperidine derivative.49... 

Some examples of three-membered chelate rings containing N and S are known. iV-Sulfinyl-aniline, and substituted forms, has been shown to bond in this manner to give chelates of the type (43) with a number of metal ions in low oxidation states. Such compounds have been reported with Ni0,150 with Rh1 and Ir1,151 and with Fe0.152 A similar bonding mode has been demonstrated for 4-(methyl)thionitrosobenzene in a bridged adduct of general form [Fe2(CO)6L].153... 

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