Some FTSE 100 dividend stocks can truly turbocharge a passive income portfolio. For instance, Legal & General (LSE:LGEN) shares are among the top five highest-yielding stocks in the index. This makes them an obvious candidate for dividend investors to consider.

But how much would I need to invest to secure £1,200 in annual dividend income? And what are the risks and opportunities potential investors should be aware of?

Let’s explore.

Big dividend payments

Currently, Legal & General shares have a dividend yield that’s just under 9.2%. Compare that to the FTSE 100 average of 3.6% and it’s clear the financial services provider is streets ahead of its Footsie counterparts.

As I write, the Legal & General share price stands at £2.25. That means I could buy 5,829 shares for a total of £13,115. Assuming dividend payouts continue, that should produce a little over £1,200 in passive income per year, giving me £100 to spend each month.

What’s more, the business recently announced a £200m share buyback programme — its first in over a decade. This is a positive move for shareholders since the total number of outstanding shares on the market falls, thus helping to boost the share price.

Chasing a high yield can be risky

So, what’s not to love about the stock’s mammoth dividend yield?

Well, there are mixed signals about the company’s dividend sustainability. After all, shareholder distributions aren’t guaranteed, so it’s crucial to look at how reliable those all-important passive income payouts are likely to be.

On the one hand, the solvency II ratio of 223% suggests a balance sheet that’s in robust health. That’s a good start.

On the other, forecast dividend cover of around one times earnings is less impressive. Ideally, I’d like cover to be twice as strong to give me comfort. As things stand, there’s a thin margin of safety for investors on this metric.

Granted, Legal & General has steadily increased its dividend since 2009. The group plans to increase the payout until 2027. But I see a risk that this ambition could come under pressure if future revenues slump.

A long-term investment

At the Motley Fool, we advocate adopting a long-term approach to investing. Looking ahead, I think the Legal & General share price is well-placed to rise in the future.

Demographics can shape a country’s destiny, and a company’s too for that matter. Aging populations across the developed world will be a defining feature of the coming decades.

This bodes well for long-term demand for Legal & General’s retirement solutions and annuities business. Indeed, the picture’s rather rosy already. First-half individual annuity sales of £1.2bn is a record for the firm and more than double those made in the previous year.

That should be weighed against an unwelcome 41% fall in post-tax profit to £223m, albeit core operating profit showed a slight improvement to reach £849m.

I also have concerns about the company’s commercial real estate exposure. The sector faces continued challenges amid growing evidence that increased remote working is cementing itself as a permanent consequence of the pandemic.

Nonetheless, trading at a forward price-to-earnings (P/E) ratio around 9.3, this dividend stock looks cheap to me right now. At this valuation, I think Legal & General shares merit serious consideration.