Ab initio MO calculations at the HF/6-31G* and MP2/6-31G* levels were carried out on five-, six-, and seven-vertex closo-heteroboranes with N, CH, P, and SiH as the heteroatoms (X, Y) capping the n-membered borocyclic ring BnHn (n = 3-5). With unsymmetrical capping groups (X not equal Y), the hydrogens of the ring bend toward the cap that is smaller in size and provide less diffuse p(pi)-orbitals. The stability and bonding in these molecules are explained using the compatibility of orbitals in overlap and the six interstitial electron rule. The five-vertex cages are seen to obey the classical 2c-2e bonding description, while the six- and seven-vertex cages are best described as three-dimensionally delocalized structures. Several thermochemical equations were derived to find the preferred combination of capping groups for a given borocyclic ring and the effect of a capping group combination as a function of ring size. In X(2)B(n)H(n), the five-vertex cage finds nitrogen as a suitable partner, while SiH as a capping group fits the seven-vertex cage perfectly. Among the closo-heteroboranes of the type XYB(n)H(n) studied, HSi-(BnHn)-N (n = 3-5) is predicted to be the most preferred combination.