The bonding between noble gas and noble metal halide like hydrogen bonding (H -bonding) motivates us to investigate the bonding mechanism and the bonding covalency in NgMX (Ng = He, Ne, Ar, Kr, Xe, Rn; M = Cu, Ag, Au; X = F, Cl, Br, I) complexes using natural bond orbital (NBO) and natural resonance theory (NRT) methods. In this study, we introduce the new resonance bonding model in H -bonding into NgMX bonding. We provide strong evidence for resonance bonding involving two important resonance structures: Ng: M X <-> Ng(+) M :X- in each of NgMX complexes, originating in the n(Ng) -> sigma*mx hyperconjugative interaction. The covalency of the bonding could be understood by the localized nature of Ng M bonds in these two resonance structures, and the degree of Ng M covalency can be quantitatively described by calculated NRT bond orders b(Ngm). Furthermore, we find that the bond order satisfies conservation of bond order, b(NgM) + b(mx) = 1, for all of the studied complexes. On the basis of the conservation of bond order and some statistical correlations, we also reveal that the Ng M bond (except He Ag and Ne Ag bonds) can be tuned by changing the auxiliary ligand X. Overall, the present studies provide new insight into the bonding mechanism and the covalency of the bonding in noble gas noble metal halides, and develop one resonance bonding model.