A systematic analysis of the thermodynamic and spectroscopic features of a vast set of data on carbonyl-like complexes present in the literature, combined with some new experimental data here reported, allowed us to attempt to single out the electrostatic sigma- and pi-contributions in the formation of carbonyl bonding for the so-called "nonclassical carbonyls" hosted inside zeolitic nanocavities. In particular, the room-temperature adsorption of CO on well-defined Cu(I) and Ag(I)-ZSM-5 systems (as testified by X-ray absorption experiments) was studied by means of the joint use of IR spectroscopy and adsorption microcalorimetry. The formation in the zeolite pores of heterogeneous [Cu(CO)(2)](+) and [Ag(CO)](+) complexes, the stoichiometry of which is in good agreement with the homogeneous nonclassical carbonyls, was monitored as a function of increasing p(CO). In the early stage of the interaction, strong and irreversibly bound monocarbonyl species characterized by (v) over tilde (COads) > (v) over tilde (COgas) are formed on Cu(I) and Ag(I) sites. Conversely, labile adducts (electrostatic in nature) are formed on Na+ and K+ sites hosted in the same zeolite pores. The zero-coverage enthalpy of CO adsorbed on Cu(I) and Ag(I) sites (-Delta(ads)H similar to 120 and similar to100 kJ/mol, respectively) is much larger than the -Delta(ads)H values measured for the two alkaline-metal adducts (similar to35 and similar to28 kJ/mol for Na+ and K+, respectively), despite the closeness of the charge/radius ratios of the two sets of metal cations [Na+ and Cu(I); K+ and Ag(I)]. The high -Delta(ads)H values and the irreversible nature of (a fraction) of the d-block metal carbonyls suggest the onset of a pi-back-donation reinforcing the carbonyl bond with respect to a plain sigma-coordination. A clear deviation from an empirical rule, which linearly correlate Delta(v) over tilde/(CO) and -DeltaH(ads) quantities for a large set of non-d/d(0)/d(10) metal carbonyls, was observed in the case of copper-and silver- carbonyls, confirming the interplay of sigma- and T-back-donation contributions for such species, otherwise defined nonclassical carbonyls. The Delta(v) over tilde (CO) versus -Delta(ads)H empirical rule was found of general validity, in that it allows to infer the enthalpy values from the blue-shift of the C-O stretching frequency (and vice-versa) in the case of non-d/d(0) metal carbonyls, whereas it allows to roughly estimate by the deviation from the line the extent of the pi-back-donation in the case of d-block metal carbonyls. Further, the spectroscopic and thermodynamic features of carbonyl species formed on (partially) reduced copper sites have shown that in the absence of strong electrostatic plus sigma-coordinative components the carbonyl bond is surprisingly weak, despite the presence of pi-back-donation.