The Square Pyramidal Configuration in a Five-Co-ordinate Nitrosyl Complex

MOST molecules which have a five-co-ordinated central atom, and the structures of which have been accurately determined, have the shape of a trigonal bipyramid; for example, PF 5 , (CH 3 )SbBr 2 , Fe(CO) 5 . Nevertheless, in their study of valency theory, Daudel and Bucher 1 predicted that a five-co-ordinate molecule may in certain circumstances have a square pyramidal configuration. In their configuration (I), the central atom A is strongly bound to four atoms B at the corners of a square, these five atoms being co-planar; this square forms the base of a pyramid, and the fifth atom C , which is more weakly bound, is at the apex. They considered that this configuration would be most stable when the lower d -orbitals of the central atom are used in forming the bonds, that is, when the hybridization is dsp 3 rather than sp 3 d .