Final answer:
To find the number of d electrons in cobalt within Na[Co(CO)2(CN)4], we calculate the oxidation state of cobalt as +3. Cobalt in its neutral state has 7 d electrons. Upon forming Co +3, it retains these 7 d electrons, making the correct answer C) 7 d electrons.
Step-by-step explanation:
To determine the number of d electrons in the cobalt (Co) atom in the metal complex Na[Co(CO)2(CN)4], first, we need to identify the oxidation state of cobalt in this complex. In the complex, CO is a neutral ligand and does not affect the charge, and each CN- ligand carries a -1 charge. With four CN- ions, the overall charge contributed by the cyanide ions is -4.
Since the entire complex is neutral and the sodium ion (Na+) contributes a +1 charge, the cobalt must have an oxidation state of +3 to balance the -4 charge from the cyanide ions and the +1 from sodium (i.e., +1 - 4 + x = 0, where x is the oxidation state of cobalt). Therefore, cobalt is in the cobalt(III) oxidation state.
Cobalt is a transition metal with an atomic number of 27, which means it has 27 protons and, in its neutral atomic state, 27 electrons as well. As a transition metal, cobalt's electrons fill the 4s orbital before they fill the 3d orbital. The electron configuration for a neutral cobalt atom is [Ar] 4s23d7. When it loses three electrons to become Co+3, it first loses the two 4s electrons and one from the 3d orbital. This leaves cobalt(III) with 7 d electrons. Therefore, the correct answer is C) 7 d electrons.