Heat of formation of co-ordination complexes of boron, aluminium and gallium

Abstract

One of the prerequisites to a full understanding of the nature of coordination compounds is a knowledge of the strength of the donor-acceptor bonds which hold them together. In many complexes the situation is complicated by the presence of several such bonds; for example the compound may be 4-, or 6-co-ordinate so that several terms go to make up the total heat of formation of the complex. For this reason it was decided to study simple 1 : I and I : 2 addition compounds rather than the usual octahedral, square, or tetrahedral complexes. In this way we hoped to be able to interpret the various energy terms more satisfactorily and so arrive at a sequence of strengths of donor-acceptor bonds between a given ligand and a series of electron acceptors, or between a given acceptor and a series of ligands. The systems we have been most interested in are those in which the electron acceptor is an element of Group III and this paper reviews the results obtained on the I : I complexes of boron trichloride and tribromide and the I : 1 and I : 2 complexes of gallium trichloride and tribromide. Two methods have been used to determine the heat of formation of a crystalline complex: equilibrium vapour pressure measurements and direct reaction calorimetry. These give the heat offormation of the solid complex and from these, the heats of formation in the gas phase can be obtained in those cases where the relevant heats of sublimation and vaporization are known. The equilibrium vapour pressure method is normally restricted to those cases where a solid complex dissociates either into a solid and a gas or into two gases. Experimentally the method simply involves synthesizing the complex in a vacuum line and then measuring its vapour pressure directly or on a spiral gauge. In this work it has been used mainly to cross-check the calorimetric measurements. As an example of its independent use, the dissociation pressure of triphenylmethyl tetrachloroborate was determined: PhaC+BCI4-(c.)~ PhaCCI(c.) + BCla(g.)