The nature of the bonding between the two M(mu-NA(not equal)) imido monomers [M = Si, Ge, Sn, Pb; Ar-# = C6H3-2,6-(C6H2-2,4,6-R-3)(2); R = Me, iPr] in the {M(mu-NAr not equal)}(2) dimer is investigated with the help of a newly developed energy and density decomposition scheme as well as molecular dynamics. The approach combines the extended transition state energy decomposition method with the natural orbitals for chemical valence density decomposition scheme within the same theoretical framework. The dimers are kept together by two a bonds and two g bonds. The a bonding has two major contributions. The first is a dative transfer of charge from nitrogen to M. It amounts to 188 kcal/mol for {Si(mu-NAr not equal)}(2), -152 kcal/mol for {Ge(p-NA not equal)}(2) with 105 kcal/mol for {Sn(p-NA?)}2, and 79 kcal/mol for {Pb(mu-NA,)}2. The second is a charge buildup within the ring made up of the two dimers. It amounts to 82 kcal/mol for M = Si with 61 kcal/mol for M = Ge and similar to-50 kcal/mol for M = Sn and Pb. We finally have pi bonding with a donation of charge from M to nitrogen. It has a modest contribution of,similar to-30 kcal/mol. The presence of isopropyl (iPr) groups is further shown to stabilize{M(mu-NA not equal)}(2) [M = Si, Ge, Sn, Pb; Ar not equal = C6H3-2,6-(C6H2-2,4,6-iPr(3))(2)] compared to the methylated derivatives (R = Me) through attractive van der Waals dispersion interactions.