Correct option is A
To find the number of valence electrons of a metal, the following steps can be used:
1.Determine the electronic configuration of the metal. This can be done by using the periodic table and filling the orbitals in order of increasing energy, starting with the lowest energy level.
2.Identify the valence electrons. The valence electrons are the outermost electrons in the atom, and they are the ones involved in chemical bonding. For transition metals, the valence electrons are the electrons in the outermost d orbitals and the s orbital.
3.Count the number of valence electrons. The number of valence electrons for transition metals is usually equal to the group number of the metal, with the exception of the first-row transition metals, which have two fewer valence electrons than their group number. For example, the group number of iron (Fe) is 8, so it has 8 valence electrons. However, since it is a first-row transition metal, it actually has 6 valence electrons.
4.Take into account any charge on the metal. If the metal has a positive charge, subtract that number from the total number of valence electrons. If it has a negative charge, add the absolute value of that number to the total number of valence electrons.
Each CO ligand donates two electrons.
Each PPh3 ligand donates two electrons.
Each metal-metal bond comprises of two electrons
In option a, electron contribution by each Ru metal: 8+4(2)+2=18
In option b, electron contribution by central Ru metal:8+2(2)+4=16
In option c, electron contribution by central Ru metal:8+2(2)+2=14
In option d, electron contribution by central Ru metal:8+2(2)+6=18. Electron contribution by other two Ru metals is 8+3(2)+3=17
Information Booster
The 18-electron rule is a chemical rule of thumb used primarily for predicting and rationalizing formulas for stable transition metal complexes, especially organometallic compounds. The rule is based on the fact that the valence orbitals in the electron configuration of transition metals consist of five (n−1)d orbitals, one ns orbital, and three np orbitals, where n is the principal quantum number. These orbitals can collectively accommodate 18 electrons as either bonding or non-bonding electron pairs. This means that the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period, lending stability to the complex. Transition metal complexes that deviate from the rule are often interesting or useful because they tend to be more reactive.
Two models are routinely used to count electrons, the so-called neutral-ligand method (sometimes called the covalent method) and the donor-pair method (sometimes known as the ionic method).
Neutral-ligand method: For the sake of counting electrons, each metal atom and ligand is treated as neutral. We include in the count all valence electrons of the metal atom and all the electrons donated by the ligands. If the complex is charged, we simply add or subtract the appropriate number of electrons to the total. The advantage of the neutral-ligand method is that, it is trivial to establish the electron count. The disadvantage, however, is that the method overestimates the degree of covalence and thus underestimates the charge at the metal. Moreover, it becomes confusing to assign an oxidation number to a metal, and meaningful information on some ligands is lost.
