Metal nitrides are rich in a variety of chemical bond nature depending on the kind of metals. The bond is ionic in alkali and alkaline earth metal nitrides. It is covalent in III and IV group metal nitrides. Metallic nature appears in transition metal nitrides. Double metal nitrides can be prepared by reactions of metal nitrides having different bond nature. High lithium ion conductivity was observed in Li₃AlN₂, Li₈SiN₄ etc, with a moderate stability against applied voltage in a combination of ionic and covalent metal nitrides. Low dimensional crystal was also formed in CaNiN having [NiN]^2-_∞  chain in combining ionic and transition metal nitrides. A number of multinary nitrides will come up in near future by a development of new preparation method. Stability of the chemical bond has not yet been quantitatively evaluated on these varieties of metal nitrides. Formation enthalpy was measured especially on the metastable iron nitrides. Very unstable Fe₁₆N₂, which has been expected to show a giant magnetization, was prepared by a very low temperature nitridation of iron metal nano-powder because of its very small formation enthalpy. Thermochemical data will be required to develop new nitrides. Interface reaction was also found out in rf-sputter deposited Fe/AlN multilayered thin films. Iron nitride with a large magnetization was formed at the interface. It can not be fully understood by a classical thermodynamics. The deposition process was simulated by a dynamical quantum chemical calculation. Hybridization of metal nitrides is a very promising material research fields. There are many open questions in this field.