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From the chemical structure of acac- compared to NH3 and en, is acac- a stronger or weaker ligand strength? How and why do we know this?

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Ligands that produce a large splitting are called strong field ligands, and those that produce a small splitting are called weak field ligands. So when crystal spittling occurs and of the 2 ligands, the one which causes greater splitting is a strong field ligand. So the strength of the ligand can be found by experimental ways . Or you to be able to decide how a ligand becomes strong field or weak field, you need to know and understand LIGAND FIELD THEORY. Whether a ligand produces a "strong" or a "weak" field was initially determined experimentally, which resulted in the so-called spectrochemical series, which lists ligands according to their field strengths. After that one can make facts and statements out of the experiments .

An easy way to identify this: The ligands in which the donor atom is Nitrogen and carbon is a strong field ligand and the ligand in which the donor atom is a halogen or oxygen is a weak field ligand. So in acac the donating atoms are Oxygen making it a weak field whereas in NH3 the donating is N making it strong field. The reason being that halogens have vacant d orbitals in their valence shell, and therefore their capacity to form a coordinate-covalent bond (by donating an electron pair) with the metal atom is relatively low, compared to ligands having pi bonds such an CN-, CO etc.

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