The Nature of Bond Orbitals in Quadricovalent Complexes of Transition Elements

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Abstract
In quadricovalent complexes of transition elements (NiII, PdII, PtII, AuIII, AgII, CuII) the central atom uses four s or d orbitals of its outer shell for unshared electron pairs and two s or d and two p orbitals for bond formation. The problem here is to find the strongest possible four hybrid bond orbitals obtained by combining the two s—d and the two p orbitals available for bond formation. The strongest bond orbitals thus obtained are directed from the middle towards the four corners of a square, as it is expected from experimental evidence; they are composed of 4/9 s, 14/9 d, and two p orbitals, the strength being 2.943. These strongest possible orbitals are presumably used in complex of NiII, PtII, AuIII, and CuII, but probably not in complexes of PdII and AgII; in the latter two cases there must be assumed square bond orbitals of composition snd2−np2 with n somewhere between 1 and 4/9, i.e., orbitals of character between that of the strongest square bond orbitals of composition s4/9d14/9p2 mentioned above and that of sdp2 square orbitals discovered by Pauling.