Comment button wordpress
2 issues - Your syntax is incorrect in that you shouldn't be using the PHP opening and closing tags, you are adding to the the variable $sharing_content so your syntax should be;
$sharing_content .='<a style="margin-left:2px; font-weight:bold;" class ="comentar" href="'. comments_link() .'">Add a comment</a>';The first link to the comments for the current post should work. However, your second link won't work at all, you've just stuck wp_list_comments(); in as the href value. wp_list_comments(); would be used to display all the comments of the current post in a template, not to link to them. I'd suggest that your href value should be the post's url, the comments will be viewable there;
$sharing_content .= '<a style="margin-left:5px; font-weight:bold;" class ="comentar" href="'. the_permalink() .'">View comments</a>';