A Tetraphosphane Imine Ligand Which Enforces Square‐Pyramidal Coordination

The tetrapodal pentadentate NP 4 ligand 2,6‐C 5 H 3 N[CMe(CH 2 PPh 2 ) 2 ] 2 , which is readily prepared from the reaction of the corresponding tetrabromide with KPPh 2 , forms mononuclear complexes with nickel( II ) and cobalt( II ). In the case of nickel( II ), the coordination mode adopted by the ligand is sensitive to the nature of the counterion: Use of NiCl 2 gives a chloro complex as one of the products, which, as determined by X‐ray crystallography, has a central ion coordinated by three phosphane and one chloro substituent in a distorted square planar fashion; by contrast, the reaction with Ni(BF 4 ) 2 gives a complex of approximate square‐pyramidal geometry as the only product, in which all five atoms of the NP 4 set are coordinated to the nickel ion, despite the steric bulk of the eight phenyl substituents. The same type of complex is obtained with Co(BF 4 ) 2 and Co(ClO 4 ) 2 . Attempts to increase the coordination number of either complex beyond 5 have been unsuccessful. In particular, the cobalt( II ) complex (17 valence electrons) is practically inert to reagents such as O 2 , NO, NO + , I 2 , Cl 2 or H 2 . Full spectroscopic details for the complexes, as well as X‐ray structure analyses, are reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)