In the electronic spectrum of [IrBr 6 ]2− , the number of charge transfer band(s) and their origin are, respectively

Question

In the electronic spectrum of [IrBr 6 ]2&minus; , the number of charge transfer band(s) and their origin are, respectively

Prepp Team · Accepted Answer

Charge Transfer Bands in [IrBr$_6$]$^{2-}$ The question asks about the number and origin of charge transfer bands in the electronic spectrum of the complex ion [IrBr$_6$]$^{2-}$. Understanding Charge Transfer Transitions Charge transfer (CT) bands arise from transitions where an electron moves from orbitals that are primarily centered on the ligand to orbitals primarily centered on the metal (Ligand to Metal Charge Transfer, LMCT) or vice versa (Metal to Ligand Charge Transfer, MLCT). The energy of these transitions depends on the relative energies of the donor and acceptor orbitals. For LMCT to occur, the metal typically needs to be in a relatively high oxidation state and/or the ligand needs to have readily available electrons (e.g., from &pi; donation). For MLCT to occur, the metal typically needs to be in a low oxidation state and the ligand needs to have low-lying empty &pi;* orbitals. Electronic Structure of [IrBr$_6$]$^{2-}$ The metal is Iridium (Ir). The complex has a charge of -2. Since bromide (Br$^-$) has a charge of -1, the oxidation state of Ir is calculated as: Ir + 6 $\times$ (-1) = -2, which gives Ir = +4. Iridium (Ir) is a 5d element. Ir in the +4 oxidation state has a d$^5$ configuration (neutral Ir is 5d$^7$6s$^2$, losing 4 electrons gives 5d$^5$). The ligand is Bromide (Br$^-$), which is a &sigma;-donor and a &pi;-donor. The complex is octahedral ($O_h$ symmetry). In an octahedral field, the metal d orbitals split into lower energy t$_{2g}$ and higher energy e$_{g}$ levels. For 5d metals, the crystal field splitting energy ($\Delta_o$) is large, leading to low-spin complexes. Thus, the 5 d$^5$ configuration is $ (t_{2g}^*)^5 (e_{g}^*)^0 $, meaning the t$_{2g}^*$ level is partially filled (5 electrons) and the e$_{g}^*$ level is empty. Note that the t$_{2g}$ and e$_{g}$ levels are antibonding MOs primarily metal in character, denoted t$_{2g}^*$ and e$_{g}^*$ respectively, when considering interactions with ligand orbitals in the MO diagram. Ligand and Metal Orbitals Involved in Charge Transfer The relevant ligand orbitals for LMCT are the filled bonding &sigma; and bonding/non-bonding &pi; orbitals. The relevant metal orbitals for LMCT are the empty or partially filled antibonding orbitals derived from the metal d orbitals, which are the t$_{2g}^*$ and e$_{g}^*$ levels. Since Ir(IV) is a high oxidation state metal, LMCT transitions (electron moving from ligand to metal) are expected to be observed. The question specifically asks about bands originating from the ligand &sigma; orbitals. The ligand &sigma; bonding orbitals are lower in energy than the ligand &pi; bonding/non-bonding orbitals. Electrons can be excited from the filled ligand &sigma; bonding orbitals to the available metal-based antibonding orbitals. The lowest energy metal-based acceptor orbitals are the t$_{2g}^*$ (partially filled) and e$_{g}^*$ (empty) orbitals. Therefore, two primary LMCT transitions originating from ligand &sigma; orbitals are expected: Transition from ligand &sigma; orbitals to the t$_{2g}^*$ metal orbitals: $\sigma \rightarrow t_{2g}$. This transition moves an electron from a filled &sigma; MO to a partially filled t$_{2g}^*$ MO. Transition from ligand &sigma; orbitals to the e$_{g}^*$ metal orbitals: $\sigma \rightarrow e_{g}$. This transition moves an electron from a filled &sigma; MO to an empty e$_{g}^*$ MO. Since the e$_{g}^*$ level is higher in energy than the t$_{2g}^*$ level, the $\sigma \rightarrow e_{g}$ transition will be at a higher energy (shorter wavelength) than the $\sigma \rightarrow t_{2g}$ transition. Conclusion on Charge Transfer Bands Based on the MO picture for an octahedral complex with &pi;-donating ligands like Br$^-$ and a d$^5$ low-spin metal like Ir(IV), two distinct LMCT bands originating from the ligand &sigma; orbitals are expected, corresponding to transitions into the t$_{2g}^*$ and e$_{g}^*$ metal-based levels. Comparing this with the given options: Option Number of bands Origin and Transitions Match? 1 Two Ligand &rarr; metal ($\sigma \rightarrow t_{2g}$ and $\sigma \rightarrow a_{1g}^*$) Partially matches the number and first transition, but the second transition to $a_{1g}^*$ (from metal s orbital) is less commonly discussed as the primary &sigma; LMCT recipient compared to the d-block e$_{g}^*$ in this context. 2 One Ligand &rarr; metal ($\sigma \rightarrow e_g$) Incorrect number of bands. 3 Two Ligand &rarr; metal ($\sigma \rightarrow t_{2g}$ and $\sigma \rightarrow e_g$) Matches the number of bands and the expected transitions to the metal d-derived antibonding levels. 4 One Ligand &rarr; metal ($\sigma \rightarrow t_g$) Incorrect number of bands and uses an unusual symmetry label. Therefore, there are two charge transfer bands originating from ligand &sigma; orbitals, corresponding to the transitions $\sigma \rightarrow t_{2g}$ and $\sigma \rightarrow e_g$.

Prepp Team · Answer

Two, ligand &rarr; metal (&sigma; &rarr; t 2g and &sigma; &rarr; a 1g *)

Prepp Team · Answer

One, ligand &rarr; metal (&sigma; &rarr; e g )

Prepp Team · Answer

One, ligand &rarr; metal (&sigma; &rarr; t g )

Charge Transfer Bands in [IrBr\(_6\)]\(^{2-}\)

Understanding Charge Transfer Transitions

Electronic Structure of [IrBr\(_6\)]\(^{2-}\)

Ligand and Metal Orbitals Involved in Charge Transfer

Conclusion on Charge Transfer Bands

Important Questions from Spectral

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Option	Number of bands	Origin and Transitions	Match?
1	Two	Ligand → metal (\(\sigma \rightarrow t_{2g}\) and \(\sigma \rightarrow a_{1g}^*\))	Partially matches the number and first transition, but the second transition to \(a_{1g}^\) (from metal s orbital) is less commonly discussed as the primary σ LMCT recipient compared to the d-block e\(_{g}^\) in this context.
2	One	Ligand → metal (\(\sigma \rightarrow e_g\))	Incorrect number of bands.
3	Two	Ligand → metal (\(\sigma \rightarrow t_{2g}\) and \(\sigma \rightarrow e_g\))	Matches the number of bands and the expected transitions to the metal d-derived antibonding levels.
4	One	Ligand → metal (\(\sigma \rightarrow t_g\))	Incorrect number of bands and uses an unusual symmetry label.