A tetraphosphane imine ligand which enforces square-pyramidal coordination

The tetrapodal pentadentate NP4 ligand 2,6-C5H3N[CMe(CH2PPh2)(2)](2), which is readily prepared from the reaction of the corresponding tetraromide with KPPh2, 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 NiCl2 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(BF4)(2) gives a complex of approximate square-pyramidal geometry as the only product, in which all five atoms of the NP4 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(BF4)(2) and Co(ClO4)(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. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)