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CHEM 236W (1)
Chapter

The Coordination, Synthesis, and Isomerization of Nitro- and Nitritopentaaamminecobalt(III)Chloride formal lab 236W.doc

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Department
Chemistry
Course Code
CHEM 236W
Professor
John Canal

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Introduction: Ligands which have the ability to bond to a metal in more than one way are called ambidentate ligands. The nitrite ion (NO ) is one2such example; it may bond via the nitrogen or oxygen atom. When this ion bonds to the metal through the nitrogen atom, it is referred to as a ‘nitro’ (NO ) g2oup. Similarly, if the nitrite is bonded through the oxygen ion, it is referred to as a ‘nitrito’ (ONO) group. In a compound, these two groups exist in equilibrium where the more stable nitro form is dominant. Figure 1: Resonance Structures of the Nitrite ion. 1 Figure 2: Nitro and Nitrito Isomers. 1 To study an ambidentate ligand, Nitritopentaamminecobalt(III)Chloride ([Co(ONO)(NH ) ]Cl 3 5ill 2e synthesized by a double replacement reaction of Cobalt(III)pentaammine chloride ([CoCl(NH ) ]Cl ) and 3 5ium2nitrate. To observe the isomerization of the nitrito group, [Co(ONO)(NH ) ]Cl was a3 5wed 2o isomerize in an oven for two days and synthetic Nitropentaamminecobalt(III)Chloride ([Co(NO ) 2 (NH )3 5l ) 2as made by dissolving [Co(ONO)(NH ) ]Cl in a hot 3 5utio2. Procedure and Observations: Preparation of Nitritopentaamminecobalt(III)Chloride ([Co(ONO)(NH ) ]Cl ): 3 5 2 Cobalt(III)pentaammine chloride ([CoCl(NH ) ]Cl ) (2.3 5g) 2as dissolved in stirred, boiling water (40mL) and ammonia (NH ) (14M) (4m3) solution. The hot, dark violet solution was filtered by gravity filtration and cooled in an ice bath. Hydrochloric acid (HCl) (6M) was added to the solution until the pH turned from 10.04 to 4.97. Sodium nitrate (NaNO ) (2.531g) and HCl (6M) (2.5mL) was added to the red solution which precipitated bright orange crystals. These crystals were isolated on a Buchner funnel and washed twice with water and ethanol. The product was left to air dry for fifteen minutes and an infrared spectroscopy (IR) was taken in a potassium bromide (KBr) pellet. This process resulted in 2.190 grams of Co(ONO)(NH ) ]Cl being 3 5 2 synthesized with a 83.9% yield. Preparation of Nitropentaamminecobalt(III)Chloride ([Co(NO )(NH ) ]Cl ): 2 3 5 2 [Co(ONO)(NH ) ]Cl 3 59942) was dissolved in a boiling water (25mL) and ammonia (14M) (1mL). As this solution was being cooled, HCl (6M) (10mL) was added drop wise. The yellow-brown [Co(NO )(NH ) ]Cl2crysta3 5were2filtered by suction and washed with ethanol (100%) (12mL). The product was left to air dry for twenty five minutes. This process resulted in 0.674 grams of ([Co(NO )(NH ) ]Cl 2eing sy3 5esi2ed with a 67.8% yield. Isomerization of [Co(ONO)(NH ) ]Cl to 3 5(NO 2(NH ) ]Cl : 2 3 5 2 The remaining [Co(ONO)(NH ) ]Cl was 3 5ed 2or two days in an oven at 110°C. This gave rise to a pale green product. Results: Balanced Equation: [CoCl(NH ) ]3 5 2(aq) NaNO 3(aq)→ [Co(ONO)(NH ) ]Cl 3 5 2(s) AgCl (aq) [Co(ONO)(NH ) ]Cl + Δ → [Co(NO )(NH ) ]Cl 3 5 2(aq) 2 3 5 2(s) [Co(ONO)(NH ) ]Cl 3 5 2(s) Δ → [Co(NO )(NH ) 2l]Cl 3 4 (s)* **see discussion on page Limiting Reagent: -1 [CoCl(NH ) ]C3 5 2 2.501g / 250.5g· mol * 1 / 1 = 0.0100 moles -1 NaNO : 3 2.511g / 69g· mol * 1 / 1 = 0.0364 moles Because [CoCl(NH ) ]Cl h3 5the 2east number of moles, it is limiting. Theoretical Yield: 0.0100 moles of [CoCl(NH ) ]Cl * 13 5lecu2e of [CoCl(NH ) ]Cl / 1 molecule o3 5 2 [Co(ONO)(NH ) ]Cl *3 51g ·2mol = 2.61 grams of [Co(ONO)(NH ) ]Cl . 3 5 2 -1 -1 0.994 g [Co(ONO)(NH ) ]Cl / 23 5 · m2l * 261g · mol = 0.994 g of [Co(NO ) 2 (NH ) 3 5 . 2 Actual Yield: [Co(ONO)(NH ) ]Cl :3 5 2(s) 2.190 g [Co(NO )(N2 ) ]Cl 3 5 2(s) 0.674 g Percent Yield: [Co(ONO)(NH ) ]Cl :3 5 2(s) 2.190 / 2.61 * 100% = 83.9% [Co(NO )(2H ) ]C3 5 2(s) 0.674 / 0.994 * 100% = 67.8% Table 1: Notable IR peaks of [Co(ONO)(NH ) ]Cl a3 5ass2ssment Frequency (cm ) -1 Peak Assessment Reference Frequency (cm ) -1 Reference Compound 2 3303 υ asH) 3444 Gas phase NH 3 3123 υ sNH) 3337 2 Gas phase NH 3
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