Structural and Thermodynamic Characteristics of Compounds X2MYH2 (M = Al,Ga, In; X = F,Cl, Br,I; Y = N,P, As) Formed by Hydrogen Halide Elimination from Donor-Acceptor Complexes X3MYH3

Geometry and thermodynamic characteristics of complexes X₂MYH₂ (M = Al, Ga, In; X = F, Cl, Br, I; Y = N, P, As) and their components were found by the B3LYP density functional method with the LANL2DZ(d,p) basic set. The nitrogen complexes X2MNH2 have a planar structure, whereas the phosphorus and arsenic complexes are pyramidal. Upon HX elimination, the dissociation energy of the M-Y bonds considerably increases (by 150-270 kJ mol^-1), which makes the dissociation of X2MYH2 into components thermodynamically unfeasible even at temperatures about 1000°C. A linear correlation between the dissociation enthalpies of M-Y bonds in the X₃MYH₃ and X₂MYH₂ complexes was found for each central atom M, which makes it possible to estimate the dissociation enthalpies of coordination-unsaturated compounds of the Group IIIa elements from the dissociation enthalpies of their coordination-saturated analogs. The enthalpies of dimerization of X₂MYH₂ fall in the range from 40 (Y = P, As) to 260 kJ mol^-1 (Y = N), which makes the process X₃MNH₃ = [X₂MNH₂]₂ + HX with the retention of the metal-nitrogen bond more favorable than the dissociation of the initial complex into the components. Thus, dimers [X₂MNH₂]₂ can be intermediates in chemical deposition of nitrides from the gas phase of donor-acceptor complexes.