Nickel is found in a wide variety of compounds, both naturally occurring and synthetically produced. Here are some examples categorized by their type:

Naturally Occurring Minerals:

* Sulfides:

* Pentlandite (Fe,Ni)9S8 - Most common nickel ore

* Millerite NiS

* Pyrrhotite Fe1-xS

* Oxides:

* Bunsenite NiO

* Silicates:

* Garnierite (Ni,Mg)6Si4O10(OH)8 - Important nickel ore

* Arsenides:

* Gersdorffite NiAsS

* Niccolite NiAs

* Other:

* Nickel-iron alloys (such as awaruite)

Synthetic Compounds:

* Salts:

* Nickel(II) chloride (NiCl2)

* Nickel(II) sulfate (NiSO4)

* Nickel(II) nitrate (Ni(NO3)2)

* Oxides:

* Nickel oxide (NiO)

* Nickel hydroxide (Ni(OH)2)

* Carbonyls:

* Nickel carbonyl (Ni(CO)4) - Used in nickel refining and organic synthesis

* Complexes:

* Nickelocene (Ni(C5H5)2) - Organometallic compound

* Tetracyanonickelate(II) anion ([Ni(CN)4]2-) - Used in electroplating

* Alloys:

* Nickel-copper (Monel) - Corrosion-resistant alloy

* Nickel-chromium (Nichrome) - High-resistance alloy used in heating elements

* Nickel-iron (Invar) - Alloy with low thermal expansion

Other:

* Nickel-cadmium batteries

* Nickel-metal hydride batteries

* Nickel-hydrogen batteries

This list is not exhaustive, as nickel is a versatile element that can be found in many different compounds.

The specific compounds of nickel used in various applications depend on the desired properties and applications. For example, nickel-iron alloys are used in magnets due to their magnetic properties, while nickel-copper alloys are used in corrosion-resistant applications due to their resistance to oxidation.