Question

The correct statement \((\mathrm{s})\) regarding the binary transition metal carbonyl compounds is (are) (Atomic numbers: \(\mathrm{Fe}=26, \mathrm{Ni}=28\) ) (A) Total number of valence shell electrons at metal centre in \(\mathrm{Fe}(\mathrm{CO})_{5}\) or \(\mathrm{Ni}(\mathrm{CO})_{4}\) is 16 (B) These are predominantly low spin in nature (C) Metal-carbon bond strengthens when the oxidation state of the metal is lowered (D) The carbonyl \(\mathrm{C}-\mathrm{O}\) bond weakens when the oxidation state of the metal is increased

Short answer

The correct statements regarding binary transition metal carbonyl compounds are (B), (C), and (D).

Step-by-step solution

Determine valence electrons for Fe in Fe(CO)\(_5\)

Iron (Fe) has an atomic number of 26, so it has 8 valence electrons (6 from the 3d orbitals and 2 from the 4s orbitals). Each CO molecule donates two electrons to the metal. In Fe(CO)\(_5\), there are 5 CO molecules, so a total of 10 electrons are donated. Adding the 8 valence electrons from Fe and the 10 donated electrons, we get a total of 18 valence shell electrons at the metal center, not 16.

Determine valence electrons for Ni in Ni(CO)\(_4\)

Nickel (Ni) has an atomic number of 28, and thus it has 10 valence electrons (8 from the 3d orbitals and 2 from the 4s orbitals). For Ni(CO)\(_4\), there are 4 CO molecules each donating 2 electrons, summing up to 8 donated electrons. Adding the 10 valence electrons from Ni and the 8 donated electrons, we also get a total of 18 valence shell electrons at the metal center, not 16.

Assess spin nature of the compounds

Transition metal carbonyl compounds are strong field ligands and thus typically form low spin complexes. This is consistent with choice (B), making it a correct statement.

Evaluate the impact of metal's oxidation state on Metal-Carbon bond strength

Lowering the oxidation state of the metal typically increases the electron density on the metal, leading to a stronger metal-carbon bond. This is because fewer electrons are drawn away from the metal by the CO ligands, indicating that choice (C) is correct.

Assess the impact of metal's oxidation state on the carbonyl C-O bond

When the oxidation state of the metal is increased, the electron density on the metal is reduced, which allows the CO ligands to withdraw less electron density from the metal. This implies that the C-O bond becomes less polar, which in turn means it becomes weaker. Therefore, choice (D) is also a correct statement.

Key concepts

Valence Shell Electrons

Understanding the count of valence shell electrons in transition metal complexes is crucial for grasping their chemical behavior. Valence shell electrons are the outermost electrons of an atom and are pivotal in chemical bonding and reactivity. In the context of transition metal carbonyl compounds, such as Fe(CO)_{5} and Ni(CO)_{4}, counting these electrons involves considering not only the electrons from the metal but also the contributions from the attached ligands. Each carbonyl (CO) ligand donates a pair of electrons, which are considered to be shared with the metal's d-orbitals, influencing properties like bonding and reactivity. The provided exercise highlights a common misconception; the total count for these compounds is often assumed to be 16, aligning with the effective atomic number (EAN) rule. However, the step-by-step solution shows the actual number to be 18. Recognizing these details assists in a more accurate understanding of the compounds' electronic structure.

Low Spin Complexes

Low spin complexes are formed when transition metals are bonded to strong field ligands, like carbonyls, causing electron pairing in lower energy orbitals. This results in fewer unpaired electrons and, consequently, lower magnetic spin states. Let's delve into why this is important for transition metal carbonyl compounds. The strong σ-donor ability of CO causes the metal's d-orbitals to split in energy, filling the lower-energy orbitals first and minimizing the number of unpaired electrons. This gives rise to low spin complexes, which are less magnetic and have distinctive chemical and spectroscopic properties compared to high spin complexes. Recognizing whether a complex is low or high spin is vital for predicting its behavior in various chemical reactions, and as the exercise solution indicates, the low spin nature of these carbonyl compounds aligns with their strong field characteristic.

Metal-Carbon Bond Strength

The robustness of the metal-carbon bond in transition metal carbonyl compounds is largely influenced by the metal's oxidation state. A lower oxidation state means that the metal retains more electrons, which enhances the metal-to-ligand bonding. Conversely, a higher oxidation state implies that the metal shares fewer of its electrons with the bonding ligands, resulting in a weaker bond. The strength of this bond is significant as it affects the stability, reactivity, and structure of the compound. For instance, a stronger metal-carbon bond might make the compound more resistant to substitution reactions, whereas a weaker bond could render it more reactive. Grasping the relationship between oxidation states and metal-carbon bond strength helps in predicting the behavior of these complexes, aligning with the correct solution provided for the exercise.

Carbonyl C-O Bond

The bond in the carbonyl ligand (CO) is of particular interest in metal carbonyl chemistry. The C-O bond in a free CO molecule is characteristically strong and triple-bonded. When CO is coordinated to a metal, the bond's polar nature is affected; the electron density withdraws towards the metal, slightly weakening the C-O bond. This withdrawal is more pronounced when the metal is in a lower oxidation state, which leads to the C-O bond becoming stronger and shorter. The relationship between the metal's oxidation state and the C-O bond is reciprocal; as the metal's oxidation state increases, the C-O bond becomes weaker, as fewer electrons are available from the metal for back-bonding. Understanding this interplay is crucial for deducing the infrared spectroscopic properties of these compounds, as stretching frequencies of the C-O bond are directly influenced by these factors.

Oxidation State Impact

The oxidation state of the central metal in a carbonyl compound has a profound impact on the compound's overall properties and stability. It determines the electron density around the metal center, influencing how electrons are distributed in metal-ligand bonds. Compounds with metals in a high oxidation state are generally more electron-deficient and therefore more prone to undergo reduction or nucleophilic attack. In contrast, those with metals in a lower oxidation state are more electron-rich and can form stronger bonds with ligands, as well as potentially acting as electrophiles. Recognizing the effect of oxidation states not only facilitates comprehension of the electronic structures but also aids in predicting the reactivity patterns and stability of carbonyl compounds, consistent with the explanations provided in the exercise solution.