## 7 Regional HSAB versus Complementary Coordinations  
One of the still problematic issues in reactivity theory is the most favorable mutual arrangement of the acidic and basic parts of molecular reactants in donor-acceptor systems. It appears that the global HSAB preference is no longer valid regionally, in interactions between fragments of reactants, where the complementarity principle [108,109] establishes the preferred arrangement between the acidic and basic parts of both substrates. These more subtle reactivity preferences result from the induced electron flows in reactive systems, reflecting responses to the primary displacements in molecular complexes. Such flow patterns can be diagnosed, estimated, and compared using either the energetical or information reactivity criteria defined above. Consider the reactive A-B complex consisting of the basic reactant B = (\(a_{\text{B}}\,|\,\ldots\,|\,b_{\text{B}}\)) \(\equiv\) (\(a_{\text{B}}\,|\,b_{\text{B}}\)) and the acidic substrate A = (\(a_{\text{A}}\,|\,\ldots\,|\,b_{\text{A}}\)) \(\equiv\) (\(a_{\text{A}}\,|\,b_{\text{A}}\)), where \(a_{\text{X}}\) and \(b_{\text{X}}\) denote the acidic and basic parts of X, respectively. The acidic (electron acceptor) part is relatively hard, i.e., less responsive to external perturbations, exhibiting lower values of the fragment FF descriptor, while the basic (electron donor) fragment is relatively soft and more polarizable, as reflected by its higher density or population response descriptors. The acidic part \(a_{\text{X}}\) exerts an electron-accepting (stabilizing) influence on the neighboring part of another reactant Y, while the basic fragment \(b_{\text{X}}\) produces an electron-donor (destabilizing) effect on a fragment of Y in its vicinity. There are two ways in which both reactants can mutually coordinate in the reactive complexes. In the complementary (\(c\)) arrangement of Figure 5,  
\[\text{R}_{c}\equiv\left[ \begin{array}{c}a_{\text{A}}-b_{\text{B}}\\ b_{\text{A}}-a_{\text{B}}\end{array} \right], \tag{78}\]  
the reactants orient themselves in such a way that geometrically accessible \(a\)-fragment of one reactant faces the geometrically accessible \(b\)-fragment of the other substrate. This pattern follows from the maximum complementarity (MC) rule of chemical reactivity [108,109], which reflects an electrostatic preference: an electron-rich (repulsive, basic) fragment of one reactant prefers to face an electron-deficient (attractive, acidic) part of the reaction partner. In the alternative regional HSAB-type structure of Figure 6,  
\[\text{R}_{\text{HSAB}}\equiv\left[ \begin{array}{c}a_{\text{A}}-a_{\text{B} }\\ b_{\text{A}}-b_{\text{B}}\end{array} \right], \tag{79}\]  
the acidic (basic) fragment of one reactant faces the like-fragment of the other substrate. The complementary complex, in which the \"excessive\" electrons of \(b_{\rm X}\) are placed in the attractive field generated by the electron \"deficient\" \(a_{\rm Y}\), is expected to be favored electrostatically since the other arrangement produces regional repulsions between two acidic and two basic sites of reactants. Additional rationale for this complementary preference over the regional HSAB alignment comes from examining charge flows created by the dominating shifts in the site chemical potential due to the presence of the (\"frozen\") coordinated site of the nearby part of the reaction partner. At finite separations between the two subsystems, these displacements trigger the polarizational flows [P\({}_{\rm X}\)] shown in Figures 5 and 6, which restore the internal equilibria in subsystems, initially displaced by the presence of the other reactant. In R\({}_{\rm c}\), the harder (acidic) site \(a_{\rm Y}\) initially lowers the chemical potential of the softer (basic) site \(b_{\rm X}\), while \(b_{\rm Y}\) rises the chemical potential level of \(a_{\rm X}\).