Species with the highest magnetic moment:
The magnetic moment of a species is a measure of its ability to be influenced by a magnetic field. It is a property that arises from the spinning of electrons within an atom or molecule. The highest magnetic moment value (spin-only value) among the given options is found in option A, VCl64-.

Explanation:
To determine the species with the highest magnetic moment, we need to consider the number of unpaired electrons present in each species. More unpaired electrons result in a higher magnetic moment.

Let's analyze each option to determine the number of unpaired electrons and hence the magnetic moment:

a) VCl64-
- Vanadium (V) has an atomic number of 23, and its electronic configuration is [Ar] 3d3 4s2.
- In the VCl64- complex, each chloride ion donates one electron, resulting in the oxidation state of +4 for vanadium.
- The electronic configuration of V4+ is [Ar] 3d1, with one unpaired electron.
- The highest spin-only value for one unpaired electron is √(n(n+2)), where n is the number of unpaired electrons.
- Therefore, the magnetic moment for V4+ is √(1(1+2)) = √3.

b) (n5-C5H5)2Cr
- Chromium (Cr) has an atomic number of 24, and its electronic configuration is [Ar] 3d5 4s1.
- In the (n5-C5H5)2Cr complex, each cyclopentadienyl (C5H5-) ligand donates one electron, resulting in the oxidation state of +2 for chromium.
- The electronic configuration of Cr2+ is [Ar] 3d4, with four unpaired electrons.
- The highest spin-only value for four unpaired electrons is √(4(4+2)) = √24.

c) [Co(NO2)6]3-
- Cobalt (Co) has an atomic number of 27, and its electronic configuration is [Ar] 3d7 4s2.
- In the [Co(NO2)6]3- complex, each nitro (NO2-) ligand donates one electron, resulting in the oxidation state of +3 for cobalt.
- The electronic configuration of Co3+ is [Ar] 3d6, with four unpaired electrons.
- The highest spin-only value for four unpaired electrons is √(4(4+2)) = √24.

d) [Ni(EDTA)]2-
- Nickel (Ni) has an atomic number of 28, and its electronic configuration is [Ar] 3d8 4s2.
- In the [Ni(EDTA)]2- complex, the EDTA ligand donates six electrons, resulting in the oxidation state of +2 for nickel.
- The electronic configuration of Ni2+ is [Ar] 3d8, with no unpaired electrons.
- Therefore, the magnetic moment for Ni2+ is 0.

Comparing the magnetic moments calculated for each option, we can conclude that option A, VCl64-, has the highest magnetic moment value of √