Co-ordination compounds of trimethylgallium with all the trimethyl derivatives of the Group VB elements from nitrogen to antimony have been prepared ; trimethylbismuth does not react. The donor properties of the VB elements, as measured by the dissociation of these co-ordination compounds, diminish regularly in the sequence N > P > As > Sb > Bi. A similar series of compounds between trimethylgallium and the dimethyl derivatives of the Group VIB elements (from 0 to Te) has also been prepared; the variation of donor properties to trimethylgallium in this group is irregular, viz., 0 > Se > S = Te. Towards trimethylaluminium the donor properties of the Group VIB elements diminish regularly from oxygen to tellurium. This contrast between trimethylgallium and trimethylaluminium, together with a review of the other co-ordination compounds of the Group VIB elements, suggests that double bonding by means of d orbitals plays an important part in determining the stability of the co-ordinate link. The co-ordinating properties of ethers are discussed, and some complexes between silver iodide and dimethyl selenide and telluride are described.THE elements of Group IIIB, viz., boron, aluminium, gallium, indium, and thallium, show a remarkable variation in their electron-acceptor properties. Compounds of all these elements in their tervalent states tend to increase their covalency to four, or sometimes six, by co-ordination with donor molecules. Boron, aluminium, and gallium exhibit this tendency to such a degree that, in some of their compounds, the element contrives to increase its covalency above three without acquiring an electron from a donor molecule; these compounds are the well known " electron-deficient " molecules exemplified by diborane B2H6, trimethylaluminium A12Me6, and digallane Ga,H,, in which the total number of valency electrons is less than twice the number of valency bonds (see Coates, Quart. Reviews, 1950, 4, 217).