Transition metal complexes of various substituted phosphine oxides and arsine oxides have been studied.

The preparation of tertiary phosphine oxide and arsine oxide complexes of cobalt (II) is reported and comparative spectral and magnetic data presented for Me₃PO and Me₃AsO tetrahedral complexes of the type [CoL₄] (ClO₄)₂ and CoL₂x₂ (X= Cl,Br). These results have been compared with those obtained for the analogous Me₃PS and Me₃AsS complexes, and they indicate that higher metal-ligand covalency is achieved in the M-S bond. Evidence for a distinct difference between P-O-M and As-O-M bonding was obtained from a study of electronic spectral band profiles.

Ph₂MePO and Ph₂MeAsO form a range of isomorphous five coordinate square pyramidal complexes of composition [ML₄(ClO₄)]ClO₄. Infrared spectral studies indicate that in the copper complexes and [Fe(Ph₂MeAsO)₄(ClO₄)]ClO₄ axial (perchlorate) bonding is weak and M-O(XPh₂Me) bonding is correspondingly stronger.

Evidence is presented to show that the corresponding Ph₃PO and Ph₃AsO complexes of the same composition have essentially the same square pyramidal structure. The electronic spectra of the cobalt and nickel perchlorate complexes have been correlated with energy level diagrams for square pyramidal chromophores.

A range of five coordinate [ML₅]²+ and [ML₄(ClO₄)]ClO₄ complexes of Me₃PO and Me₃AsO have been prepared. Corresponding arsine oxide and phosphine oxide complexes are isomorphous.

The crystal structure of Ni(Me₃AsO)₅₂ has been determined and the cation has been found to be a distorted square pyramid.

A limited number of complexes of the bidentate ligand, o-phenylene bis (dimethylarsine oxide) (diarsine dioxide), have been prepared. The halide complexes, CoLX₂, are tetrahedral and possibly polymeric.

The crystal structure of Zn(C₆H₄(Me₂AsO)₂)₂₂ which is isomorphous to the analogous cobalt complex, has been determined. The complex is dimeric, and the zinc atom is at the centre of a trigonal bipyramidal arrangement of oxygen atoms.