The results of an energy decomposition analysis of various classes of donor–acceptor complexes of transition metals and main-group elements are discussed. It is shown that the nature of the chemical bond can be quantitatively identified in terms of Pauli repulsion, electrostatic attraction and covalent bonding. The covalent and electrostatic contributions to the interatomic attraction can be precisely given by using a well defined partitioning method in conjunction with accurate quantum chemical calculations of the geometries and bond energies. This is shown for six classes of donor–acceptor complexes: (a) transition metal carbonyl complexes; (b) transition metal complexes with Group-13 diyl ligands ER (E=BTl); (c) transition metal complexes with phosphane ligands (CO) 5TMPX 3 (TM=Cr, Mo, W; X=H, Me, F, Cl); (d) main group complexes with phosphane ligands X 3BPY 3 and X 3AlPY 3 (X=H, F, Cl; Y=F, Cl, Me, CN); (e) transition metal metallocene complexes Fe(η 5-E 5) 2 and FeCp(η 5-E 5) (E=CH, N, P, As, Sb); (f) main group metallocenes ECp 2 (E=BeBa, Zn, SiPb) and ECp (E=LiCs, BTl).