Which of the following complexes shows fac-mer isomerism?

Isomerism in coordination compounds is rich and diverse, arising from the various ways in which the constituents of a coordination compound can be arranged in space or distributed between the inner coordination sphere and the outer sphere. The major types are: Structural isomers (constitutional isomers): same molecular formula but different connectivity. Ionisation isomers: differ in which ions are inside and outside the coordination sphere. [Co(NH3)5Br]SO4 (bromide inside, sulfate outside — ionises to give Br- and [Co(NH3)5Br]SO4 → adds AgNO3 gives no precipitate immediately, but adds BaCl2 gives white BaSO4 precipitate) versus [Co(NH3)5SO4]Br (sulfate inside, bromide outside — ionises to give Br- and SO4^2- outside → adds AgNO3 gives cream AgBr precipitate immediately). Linkage isomers: formed when a ligand can bond to the metal through different donor atoms (ambidentate ligands). Thiocyanate (SCN-) can bond as M-SCN (S-bonded, thiocyanate) or M-NCS (N-bonded, isothiocyanate). Nitrite (NO2-) can bond as M-NO2 (N-bonded, nitro) or M-ONO (O-bonded, nitrito). [Co(NH3)5NO2]2+ (yellow, nitro, N-bonded) vs [Co(NH3)5ONO]2+ (red, nitrito, O-bonded) — the nitrito form is thermodynamically unstable and converts to the nitro form on standing. Coordinate isomers: when a complex contains both cationic and anionic coordination complex ions, different distributions of ligands between the two metal centres give coordination isomers.

Geometric isomerism arises when ligands can occupy different positions relative to each other in space. For square planar complexes (MA2B2 type, e.g., [Pt(NH3)2Cl2]): cis isomer has both A ligands on the same side (A-Pt-A angle = 90°); trans isomer has A ligands on opposite sides (A-Pt-A angle = 180°). Cis-[Pt(NH3)2Cl2] is cisplatin (used in cancer chemotherapy); trans-[Pt(NH3)2Cl2] is transplatin (pharmacologically inactive). For octahedral MA4B2 type complexes (e.g., [Co(NH3)4Cl2]+): cis isomer has the two B ligands on adjacent positions (B-M-B angle = 90°); trans isomer has B ligands directly opposite (B-M-B angle = 180°). For octahedral MA3B3 type complexes: fac isomer has three A ligands on one triangular face of the octahedron (all A-M-A angles = 90°); mer isomer has three A ligands in a meridional plane (two A ligands are trans to each other, A-M-A angle = 180°, and the third A is cis to both, A-M-A angle = 90° with each). This fac-mer distinction is unique to octahedral MA3B3-type complexes and is an important concept frequently tested in competitive examinations.

Optical isomerism arises when a complex and its mirror image are non-superimposable (the complex is chiral). In coordination chemistry, optical isomerism is most commonly found in octahedral complexes with chelating ligands. Tris-chelate complexes M(AA)3 (e.g., [Co(en)3]3+, [Cr(ox)3]3-, [Fe(ox)3]3-) are always optically active — they have right-handed (Delta, d) and left-handed (Lambda, l) helical propeller arrangements of the three bidentate ligands, which are non-superimposable mirror images. Cis-bis chelate complexes cis-[M(AA)2B2] are optically active (the cis geometry lacks a plane of symmetry); trans-[M(AA)2B2] are optically inactive (have a plane of symmetry perpendicular to the B-M-B axis). Historical significance: Alfred Werner resolved [Co{Co(NH3)4(OH)2}3]Br6, a coordination compound with NO carbon atoms, into its enantiomers in 1914 — proving that chirality is a property of three-dimensional molecular geometry, not limited to carbon-based organic compounds.

The Effective Atomic Number (EAN) rule, proposed by Sidgwick in 1927, states that stable coordination complexes tend to have the central metal surrounded by a total of 18 electrons in its valence shell (analogous to the electron configuration of the nearest noble gas). The 18-electron rule arises from the fact that in a transition metal coordination complex, the metal has 5 d-orbitals, 1 s-orbital, and 3 p-orbitals available for bonding — 9 orbitals total, accommodating 18 electrons. When the metal achieves 18 electrons, all bonding molecular orbitals are filled and all antibonding orbitals are empty, giving maximum bond stability. Examples of 18-electron complexes: [Fe(CO)5] (Fe0 has 8 electrons, 5 CO donate 10 electrons, total = 18). [Ni(CO)4] (Ni0 has 10 electrons, 4 CO donate 8 electrons, total = 18). [Co(CO)4]- (Co-1 has 10 electrons, 4 CO donate 8 electrons, total = 18). However, the 18-electron rule is not universal — many stable complexes have fewer than 18 electrons: [Ni(NH3)6]2+ has 20 electrons (Ni2+ has 8 d-electrons + 12 from 6 NH3 = 20). The rule is most strictly followed by organometallic complexes with pi-acceptor ligands (CO, alkenes, cyclopentadienyl) and not by Werner-type complexes with classical donor ligands.

IUPAC systematic nomenclature for coordination compounds follows specific rules: Name the cation (or whole complex if neutral) before the anion. Name ligands before the metal. Anionic ligands end in -o (Cl- = chlorido, CN- = cyanido, OH- = hydroxido, NO2- = nitrito-N, O2- = oxido, CO3^2- = carbonato, SO4^2- = sulfato, C2O4^2- = oxalato, en = ethane-1,2-diamine). Neutral ligands use their name: NH3 = ammine, H2O = aqua, CO = carbonyl, NO = nitrosyl, py = pyridine. Prefix for number: di, tri, tetra, penta, hexa (bis, tris, tetrakis for complex ligands or when di etc. is part of ligand name). Metal name: for cationic complex, use English name of metal followed by oxidation state in Roman numerals in parentheses. For anionic complex, use Latin/systematic root ending in -ate: Fe → ferrate, Cu → cuprate, Pb → plumbate, Au → aurate, Ag → argentate, Sn → stannate; other metals use -ate suffix on English name. Alphabetical order of ligands (not by number). Examples: [Co(NH3)6]Cl3 = hexaamminecobalt(III) chloride. K3[Fe(CN)6] = potassium hexacyanidoferrate(III) (potassium ferricyanide). [Cr(en)3]Cl3 = tris(ethane-1,2-diamine)chromium(III) chloride. Na2[PtCl4] = sodium tetrachloridoplatinate(II).

Metal carbonyl complexes, typified by [Ni(CO)4], [Fe(CO)5], and [Cr(CO)6], represent a special and fascinating class of coordination compounds where the bonding involves both sigma donation and pi backbonding in a synergistic interaction called the synergistic bonding model or Dewar-Chatt-Duncanson model. Sigma donation: CO donates electron density from its HOMO (highest occupied molecular orbital, the sigma lone pair on carbon) to an empty metal d-orbital (or hybrid orbital), forming a sigma bond. This makes CO a weak Lewis acid and transfers charge from CO to the metal. Pi backbonding (pi-backdonation): the metal donates electron density from its filled d-orbitals (the t2g set in octahedral complexes) into the empty pi* antibonding MOs of CO. This backbonding transfers charge from the metal back to CO. The net result of this synergistic interaction is that both the metal-CO sigma bond and the metal-CO pi bond are strengthened simultaneously — the charge transfer in sigma donation is partially balanced by the charge transfer in pi backbonding, stabilising the overall bonding. Evidence for pi backbonding: the C-O stretching frequency in metal carbonyls is lower than in free CO (2143 cm-1): [Cr(CO)6]: 2000 cm-1. [Fe(CO)5]: 2013 and 2034 cm-1. [Ni(CO)4]: 2057 cm-1. The lower C-O stretching frequency indicates a weaker C-O bond because pi backbonding puts electron density into the C-O pi* orbital (which is antibonding with respect to the C-O bond). Higher electron density on metal (lower oxidation state or more electron-donating co-ligands) → more backbonding → lower C-O frequency.