Oxidation number also called oxidation state is the total number of electrons that an atom either gains or loses in order to form a chemical bond with another form. [Pt(NH3)2Cl(NO2)] Let x be the oxidation state of platinum. The oxidation state of ammonia (NH3) and NO2 is zero. The oxidation state of chlorine is −1. x+2(0)+2(−1)+0‌‌=0 x−2‌‌=0 x‌‌=+2 22]+‌ [CoCl2(en)2]+ Let, y be the oxidation state of cobalt. The oxidation state of en is 0 and oxidation state of chlorine is −1. y+2(−1)+0−1‌‌=0 y−2−1‌‌=0 y−3‌‌=0 y‌‌=+3 So, oxidation number of central metal in [Pt(NH3)2Cl(NO2)] is +2 and in [CoCl2(en)2]+ is +3.